mesa/teximage: return the base internal format of the ASTC formats

mesa/teximage: return the base internal format of the ASTC formats

This is necesary to initialize the gl_texture_image struct.

From the KHR_texture_compression_astc_ldr spec:
  "Added to Section 3.8.6, Compressed Texture Images

   Add the tokens specified above to Table 3.16, Compressed Internal Formats.
   In all cases, the base internal format will be RGBA. The encoding allows
   images to be encoded with fewer channels, but this is always presented as
   RGBA to the sampler."

v2. use _mesa_is_astc_format().

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