intel/blorp: Fix blits to R8G8B8_UNORM_SRGB sRGB harder

intel/blorp: Fix blits to R8G8B8_UNORM_SRGB sRGB harder

The first fix attempt contained a nasty typo which somehow didn't get
caught in review.  It also didn't work as intended because the sRGB
conversion was happening but then throwing away all but the red channel
because it dind't know it was RGB.  Really, it's my fault for trying to
fix a bug without first writing tests.  I've now written tests and they
pass with this change. :)

Fixes: 11712b9ca17 "intel/blorp: Fix blits to R8G8B8_UNORM_SRGB"
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>