drm/fsl-dcu: deallocate fbdev CMA on unload

Free fbdev CMA using drm_fbdev_cma_fini on unload. This fixes
a warning when unloading the driver:
WARNING: CPU: 0 PID: 164 at drivers/gpu/drm/drm_crtc.c:5930 drm_mode_config_cleanup+0x204/0x208

Signed-off-by: Stefan Agner <stefan@agner.ch>