drm_plane_helper_check_state() is supposed to do things the atomic way,
so it should not be inspecting crtc->enabled. Rather we should be
looking at crtc_state->enable.

We have a slight complication due to drm_plane_helper_check_update()
reusing drm_plane_helper_check_state() for non-atomic drivers. Thus
we'll have to pass the crtc_state in manally and construct a fake
crtc_state in drm_plane_helper_check_update().

v2: Fix the WARNs about plane_state->crtc matching crtc_state->crtc
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171101201558.6059-1-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>