drm: Clear crtc->primary->crtc when disabling the crtc via setcrtc()

drm: Clear crtc->primary->crtc when disabling the crtc via setcrtc()

Keep the primary->crtc in sync with the state->crtc (also with
primary->fb and state->fb) when disabling the crtc (and thus also
the primary) via setcrtc().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-4-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>