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

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

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101184646.14753-2-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>