mesa: don't use format chooser code for glCompressedTexImage

mesa: don't use format chooser code for glCompressedTexImage

When glCompressedTexImage is called the internalFormat is a specific
format for the incoming image and the the hardware format should be
the same (since we never do format transcoding).  So use the simpler
_mesa_glenum_to_compressed_format() function.  This change is also
needed for the next patch.

Note: This is a candidate for the stable branches.