mesa: Add the format enums for BPTC-compressed images

mesa: Add the format enums for BPTC-compressed images

This adds the following four Mesa image format enums which correspond to the
four BPTC compressed texture formats:

 MESA_FORMAT_BPTC_RGBA_UNORM
 MESA_FORMAT_BPTC_SRGB_ALPHA_UNORM
 MESA_FORMAT_BPTC_RGB_SIGNED_FLOAT
 MESA_FORMAT_BPTC_RGB_UNSIGNED_FLOAT

It also updates the format information functions to handle these and the
corresponding GL enums.

v2: Also modify _mesa_get_format_color_encoding, _mesa_get_srgb_format_linear
    and _mesa_get_uncompressed_format

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>