mesa: Move _mesa_base_tex_format() from teximage to glformats files

mesa: Move _mesa_base_tex_format() from teximage to glformats files

This function will be needed as part of validating the combination of format,
type and internal format of texture pixel operations, which happens in
glformats files. Specifically, we want to be able to obtain the base format
of a resolved effective internal format, to compare it with the original
internal format passed.

Also, since this function deals solely with GL formats, it fits better in
glformats where the rest of similar format functionality rests.

The function is moved as-is, without any modification.

Cc: "11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>