drm: don't associate _DRM_DRIVER maps with a master

A driver will use the _DRM_DRIVER map flag to indicate that it wants
to be responsible for removing the map itself, bypassing the DRM's
automagic cleanup code.

Since the multi-master changes this has been broken, resulting in some
drivers having their registers unmapped before it's finished with them.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>