mesa/teximage: reuse compressed format utility functions for base_format

mesa/teximage: reuse compressed format utility functions for base_format

Reuse utility functions instead of reimplementing the same logic.

* _mesa_is_compressed_format() performs the required checking to
  determine format support in the current context.
* _mesa_gl_compressed_format_base_format() returns the base format.

As a side effect, we now check that we're in a desktop context when
determining support for the FXT1 and RGTC formats. This is in agreement
with our extension table and the glext headers.

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