drm/i915: Make the cmd parser 64bit regs explicit

Add defines for the upper halves of the registers used by the cmd
parser. Getting rid of the arithmetic with the register offset
will help in making registers type safe.

v2: s/_HI/_UDW/ (Chris)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1446839080-18732-1-git-send-email-ville.syrjala@linux.intel.com