iris: Fix fast-clears of swizzled LA formats

Resources with luminance alpha formats that are mapped to RG are
fast-cleared with the wrong clear color.

When such resources with are cleared via iris_clear_texture,
isl_color_value_unpack places channel data in the R and G channels.
convert_fast_clear_color then overwrites the G channel with R.

Delete the clear color override that's specific to luminance alpha
formats.

Enables iris to pass the "LA" case of the fcc-clear-tex piglit test.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7345>
diff --git a/src/gallium/drivers/iris/iris_clear.c b/src/gallium/drivers/iris/iris_clear.c
index a4d45a4..ebeb39a 100644
--- a/src/gallium/drivers/iris/iris_clear.c
+++ b/src/gallium/drivers/iris/iris_clear.c
@@ -148,8 +148,7 @@
    unsigned colormask = util_format_colormask(desc);
 
    if (util_format_is_intensity(format) ||
-       util_format_is_luminance(format) ||
-       util_format_is_luminance_alpha(format)) {
+       util_format_is_luminance(format)) {
       override_color.u32[1] = override_color.u32[0];
       override_color.u32[2] = override_color.u32[0];
       if (util_format_is_intensity(format))