mesa: replace gl_context->Multisample._Enabled with _mesa_is_multisample_enabled.

mesa: replace gl_context->Multisample._Enabled with _mesa_is_multisample_enabled.

This removes any dependency on driver validation of the number of
framebuffer samples.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Brian Paul <brianp@vmware.com>