mesa: improve comment on _mesa_check_disallowed_mapping(), return bool
mesa: improve comment on _mesa_check_disallowed_mapping(), return bool

The old comment was a bit terse.  Also, change the function return
type to bool.

Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler