Rename enum plane to enum i9xx_plane_id to make it clear that it only
applies to pre-SKL platforms.

enum i9xx_plane_id is a global identifier, whereas enum plane_id is
per-pipe. We need the old global identifier to index the primary plane
(and the pre-g4x sprite C if we ever expose it) registers on pre-SKL
platforms.

v2: Reorder patches
v3: s/old_plane_id/i9xx_plane_id/ (Daniel)
    Pimp the commit message a bit
    Note that i9xx_plane_id doesn't apply to SKL+
v4: Rebase due to power domain handling in plane readout
v5: Rebase due to crtc->dspaddr_offset removal
v6: s/plane/i9xx_plane/ etc. (James)

Cc: James Ausmus <james.ausmus@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117191917.11506-4-ville.syrjala@linux.intel.comReviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>