drm/i915: Enable default_phase in GCP when possible

When the video timings are suitably aligned so that all different
periods start at phase 0 (ie. none of the periods start mid-pixel)
we can inform the sink about this. Supposedly the sink can then
optimize certain things. Obviously this is only relevant when
outputting >8bpc data since otherwise there are no mid-pixel phases.

v2: Rebased due to crtc->config changes

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chandra Konduru <Chandra.konduru@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>