i965: Add support for and expose EXT_texture_sRGB_R8

i965: Add support for and expose EXT_texture_sRGB_R8

Emulate MESA_FORMAT_R_SRGB8 by using L8_UNORM_SRGB. This is possible
because component swizzling is handled based on the mesa format and,
hence, the a r001 swizzling can be used to correct the components.

Enables and makes pass (tested on Kabylake)

  dEQP-GLES31.functional.srgb_texture_decode.skip_decode.sr8.*
  dEQP-GLES31.functional.texture.filtering.cube_array.formats.sr8*

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>