mesa/formats: define the 2D ASTC formats

mesa/formats: define the 2D ASTC formats

Define the mesa formats and make changes necessary for compilation
without errors. Also add support for _mesa_get_srgb_format_linear().

v2. conform the ASTC MESA_FORMAT enums to the existing naming convention.
v3. remove ASTC cases for _mesa_get_uncompressed_format(). This function is
    only used for generating mipmaps - something ASTC formats do not support
    due to lack of online compression.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>