drm/nouveau/kms/nv50-: Use state helper instead of crtc pointer

dispnv50 references the crtc->state pointer in order to get the current
CRTC state in its atomic_check hook, which would be the old CRTC state in
the global atomic state.

Use the drm_atomic_get_old_crtc_state helper to get that state to make it
more obvious.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201102133834.1176740-1-maxime@cerno.tech