drm/atomic: Use explicit old/new state in drm_atomic_plane_check()

Convert drm_atomic_plane_check() over to using explicit old vs. new
plane states. Avoids the confusion of "what does plane->state mean
again?".

v2: Stick to the multi-stage logic in plane_switching_crtc() (Daniel)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181106191624.2207-1-ville.syrjala@linux.intel.com