drm/i915: Nuke crtc->plane

drm/i915: Nuke crtc->plane

Eliminate crtc->plane since it's pretty much a layering violation.
We can always get the plane via crtc->primary if we actually need it.

The only ugly thing left is plane_to_crtc_mapping[], but that's
still needed by the pre-g4x watermark code.

v2: Removed a misplaced comment change (Daniel)
v3: Rebase due to fbc crtc->y usage removal
v4: s/plane/i9xx_plane/ etc. (James)

Cc: James Ausmus <james.ausmus@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117191917.11506-9-ville.syrjala@linux.intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>