drm: try harder to avoid regression when merging mode bits

For QXL hw we really want the bits to be replaced as we change
the preferred mode on the fly, and the same goes for virgl when
I get to it, however the original fix for this seems to have caused
a wierd regression on Intel G33 that in a stunning display of failure
at opposition to his normal self, Daniel failed to diagnose.

So we are left doing this, ugly ugly ugly ugly, Daniel you fixed
that G33 yet?, ugly, ugly.

Tested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>