i965: Explicitely handle swizzles for MESA_FORMAT_R_SRGB8

i965: Explicitely handle swizzles for MESA_FORMAT_R_SRGB8

The format is emulated by using ISL_FORMAT_L8_SRGB, therefore we need to
force swizzles for the GBA channels. However, doing this only based on the
data type GL_RED breaks other formats, therefore, test specifically for the
format.

Fixes: c5363869d4971780401b21bb75083ef2518c12be
  i965: Force zero swizzles for unused components in GL_RED and GL_RG
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>