drm/i915: Rename graphics reset registers.

The graphics domains are listed as GRDOM in the documentation, and the
GDRST PCI config register (0xc0) is only valid on I965 and GM45.  Newer
chips (like Sandy Bridge) have a different GDRST.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>