drm/i915: Stop looking at plane->state in intel_prepare_plane_fb()

drm/i915: Stop looking at plane->state in intel_prepare_plane_fb()

Switch over to using explicit old/new planes states instead of
digging the old state out via plane->state. The main issue is that
plane->state will point to the uapi state which we generally don't
even want to look at.

Also it sets a bad example as using plane->state during commit_tail()
would be a bug. Here we're still holding the modeset locks so it's
actually safe, but best not give people bad ideas.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200110183228.8199-3-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>