drm_hwcomposer: check if the plane is being used by any crtc
if the plane is used for other crtc, then that crtc is not supported by this plane. As it's protected by the plane_switching_crtc function in the kernel drivers/gpu/drm/drm_atomic.c file
Signed-off-by: Yongqin Liu yongqin.liu@linaro.org