i965: Support MESA_FORMAT_R8G8B8X8_SRGB.

Valve games use GL_SRGB8 textures.  Instead of supporting that properly,
we fell back to MESA_FORMAT_R8G8B8A8_SRGB (with an alpha channel), which
meant that we had to use texture swizzling to override the alpha to 1.0
when sampling.  This meant shader recompiles on Gen < 7.5 platforms.

By supporting MESA_FORMAT_R8G8B8X8_SRGB, the hardware just returns 1.0
for us, so we can just use SWIZZLE_XYZW, and avoid any recompiles.  All
generations of hardware have supported the format for sampling and
filtering; we can easily support rendering by using the R8G8B8A8_SRGB
format and writing garbage to the X channel.  (We do this already for
the non-SRGB version of this format.)

This removes all remaining shader recompiles in a time demo of "Counter
Strike: Global Offensive" (32 -> 0) on Sandybridge.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87886
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>

src/mesa/drivers/dri/i965/brw_surface_formats.c

@@ -518,7 +518,7 @@ brw_format_for_mesa_format(mesa_format mesa_format)
       [MESA_FORMAT_B4G4R4X4_UNORM] = 0,
       [MESA_FORMAT_B5G5R5X1_UNORM] = BRW_SURFACEFORMAT_B5G5R5X1_UNORM,
       [MESA_FORMAT_R8G8B8X8_SNORM] = 0,
-      [MESA_FORMAT_R8G8B8X8_SRGB] = 0,
+      [MESA_FORMAT_R8G8B8X8_SRGB] = BRW_SURFACEFORMAT_R8G8B8X8_UNORM_SRGB,
       [MESA_FORMAT_X8B8G8R8_SRGB] = 0,
       [MESA_FORMAT_RGBX_UINT8] = 0,
       [MESA_FORMAT_RGBX_SINT8] = 0,
@@ -595,6 +595,9 @@ brw_init_surface_formats(struct brw_context *brw)
       case BRW_SURFACEFORMAT_R8G8B8X8_UNORM:
          render = BRW_SURFACEFORMAT_R8G8B8A8_UNORM;
          break;
+      case BRW_SURFACEFORMAT_R8G8B8X8_UNORM_SRGB:
+         render = BRW_SURFACEFORMAT_R8G8B8A8_UNORM_SRGB;
+         break;
       }
 
       rinfo = &surface_formats[render];