drm: Fix shifts which were miscalculated when converting from bitfields.

Looks like I managed to mess up most shifts when converting from bitfields. :(

The patch below works on my Thinkpad T500 (as well as on my PowerBook,
where the previous change worked as well, maybe out of luck...). I'd
appreciate more testing and eyes looking over it though.

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Tested-by: Michael Pyne <mpyne@kde.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>