mesa: Prepare for the MESA_FORMAT_* enum to be sparse.

mesa: Prepare for the MESA_FORMAT_* enum to be sparse.

To redefine MESA_FORMAT in terms of PIPE_FORMAT enums, we need to fix
places where we iterated up to MESA_FORMAT_COUNT.  I use
_mesa_get_format_name(f) == NULL as the signal that it's not an enum
value with a MESA_FORMAT.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>