drm/i915: Add per-pipe plane identifier

As I told people in [1] we really should not be confusing enum plane
as a per-pipe plane identifier. Looks like that happened nonetheless, so
let's fix it up by splitting the two into two enums.

We'll also want something we just directly pass to various register
offset macros and whatnot on SKL+. So let's make this new thing work for that.
Currently we pass intel_plane->plane for the "sprites" and just a
hardcoded zero for the "primary" planes. We want to get rid of that
hardocoding so that we can share the same code for all planes (apart
from the legacy cursor of course).

[1] https://lists.freedesktop.org/archives/intel-gfx/2015-September/076082.html



Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479830524-7882-2-git-send-email-ville.syrjala@linux.intel.com