drm/i915: Add POWER_DOMAIN_VGA

VGA registers/memory live inside the the display power well. Add a power
domain for VGA.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>