drm/i915: Cleanup enum pipe/enum plane_id/enum i9xx_plane_id in initial fb readout

drm/i915: Cleanup enum pipe/enum plane_id/enum i9xx_plane_id in initial fb readout

Use enum pipe, enum plane_id, and enum i9xx_plane_id consistently in the
initial framebuffe readout.

v2: Use old_plane_id in the ilk code
v3: s/old_plane_id/i9xx_plane_id/ (Daniel)
v4: Rebase due to GLK/CNL PLANE_COLOR_CTL alpha stuff
v5: 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/20171117191917.11506-6-ville.syrjala@linux.intel.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>