drm/amd/display: Fix misuse of plane state destroy helper

drm_plane_state is subclassed by dm_plane_state, so atomic driver should
use __drm_atomic_helper_plane_destroy_state() to destroy a state.
drm_atomic_helper_plane_destroy_state() frees drm_plane_state which is
allocated inside dm_plane_state, this is problematic.

Fixes: 95ae03a ("drm/amd/display: Create dm_plane_state.")
Signed-off-by: Xiaojie Yuan <Xiaojie.Yuan@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>