drm: atmel-hlcdc: fix atmel_hlcdc_crtc_reset() implementation

Reset crtc->state to NULL after freeing the state object and call
__drm_atomic_helper_crtc_destroy_state() helper instead of manually
calling drm_property_unreference_blob().

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>