drm/i915: Use enum plane_id for frontbuffer tracking

drm/i915: Use enum plane_id for frontbuffer tracking

Replace the ad-hoc plane indexing scheme used by the frontbuffer
tracking with enum plane_id.

The old video overlay not being part of the plane_id namespace
will just be given the high bit.

v2: Drop the unintended whitespace change (Chris)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180123183343.9181-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>