drm/vc4: Stop updating plane->fb/crtc

drm/vc4: Stop updating plane->fb/crtc

We want to get rid of plane->fb/crtc on atomic drivers. Stop setting
them.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180525185045.29689-13-ville.syrjala@linux.intel.com
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>