freedreno/a6xx: Separate stencil sysmem clear fix

We need to clear with PIPE_FORMAT_S8_UINT.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5717>
diff --git a/src/gallium/drivers/freedreno/a6xx/fd6_gmem.c b/src/gallium/drivers/freedreno/a6xx/fd6_gmem.c
index 4707eee..c43d26b 100644
--- a/src/gallium/drivers/freedreno/a6xx/fd6_gmem.c
+++ b/src/gallium/drivers/freedreno/a6xx/fd6_gmem.c
@@ -1327,6 +1327,7 @@
 			value.ui[0] = batch->clear_stencil;
 
 			struct pipe_surface stencil_surf = *pfb->zsbuf;
+			stencil_surf.format = PIPE_FORMAT_S8_UINT;
 			stencil_surf.texture = separate_stencil;
 
 			fd6_clear_surface(ctx, ring,