radeonsi: fix texture gather on stencil textures

At least on VI, texture gather doesn't work with a 24_8 data format, so
use 8_8_8_8 and a modified swizzle instead.

A bit of background: When creating a GL_STENCIL_INDEX8 texture, we select
the X24S8 pipe format because we don't support stencil-only render targets
properly. With mip-mapping this can lead to a setup where the tiling is
incompatible with stencil texturing, and a flushed stencil texture is
used. For the flushed stencil, a literal X24S8 is used because there were
issues with an 8bpp DB->CB copy.

Longer term, it would be good if we could get away from these workarounds,
i.e. properly support an S8 format for stencil-only rendering and flushed
stencil. Since stencil texturing is somewhat rare, it's not a high
priority.

Fixes GL45-CTS.texture_cube_map_array.sampling.

Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
(cherry picked from commit 3cd092c41508dde2e6259f09df1736911a828548)
diff --git a/src/gallium/drivers/radeonsi/si_state.c b/src/gallium/drivers/radeonsi/si_state.c
index 0ec34f9..876cbf6 100644
--- a/src/gallium/drivers/radeonsi/si_state.c
+++ b/src/gallium/drivers/radeonsi/si_state.c
@@ -1365,11 +1365,17 @@
 		case PIPE_FORMAT_Z16_UNORM:
 			return V_008F14_IMG_DATA_FORMAT_16;
 		case PIPE_FORMAT_X24S8_UINT:
+		case PIPE_FORMAT_S8X24_UINT:
+			/*
+			 * Implemented as an 8_8_8_8 data format to fix texture
+			 * gathers in stencil sampling. This affects at least
+			 * GL45-CTS.texture_cube_map_array.sampling on VI.
+			 */
+			return V_008F14_IMG_DATA_FORMAT_8_8_8_8;
 		case PIPE_FORMAT_Z24X8_UNORM:
 		case PIPE_FORMAT_Z24_UNORM_S8_UINT:
 			return V_008F14_IMG_DATA_FORMAT_8_24;
 		case PIPE_FORMAT_X8Z24_UNORM:
-		case PIPE_FORMAT_S8X24_UINT:
 		case PIPE_FORMAT_S8_UINT_Z24_UNORM:
 			return V_008F14_IMG_DATA_FORMAT_24_8;
 		case PIPE_FORMAT_S8_UINT:
@@ -2796,14 +2802,22 @@
 	if (desc->colorspace == UTIL_FORMAT_COLORSPACE_ZS) {
 		const unsigned char swizzle_xxxx[4] = {0, 0, 0, 0};
 		const unsigned char swizzle_yyyy[4] = {1, 1, 1, 1};
+		const unsigned char swizzle_wwww[4] = {3, 3, 3, 3};
 
 		switch (pipe_format) {
 		case PIPE_FORMAT_S8_UINT_Z24_UNORM:
-		case PIPE_FORMAT_X24S8_UINT:
 		case PIPE_FORMAT_X32_S8X24_UINT:
 		case PIPE_FORMAT_X8Z24_UNORM:
 			util_format_compose_swizzles(swizzle_yyyy, state_swizzle, swizzle);
 			break;
+		case PIPE_FORMAT_X24S8_UINT:
+			/*
+			 * X24S8 is implemented as an 8_8_8_8 data format, to
+			 * fix texture gathers. This affects at least
+			 * GL45-CTS.texture_cube_map_array.sampling on VI.
+			 */
+			util_format_compose_swizzles(swizzle_wwww, state_swizzle, swizzle);
+			break;
 		default:
 			util_format_compose_swizzles(swizzle_xxxx, state_swizzle, swizzle);
 		}