drm/i915: Add register whitelist for DRM master

These are used to implement scanline waits in the X server.

v2: Use #defines instead of magic numbers

Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>