drm/i915: Give C0DRB3/C1DRB3 a _BW suffix

These are the 965g/g45/g33 specific DRB registers. Give them
a suitable suffix so we can add their counterparts for other
platforms.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421153401.13847-4-ville.syrjala@linux.intel.com