drm: mali-dp: Turn off CRTC vblank when removing module.

When unbinding the mali-dp driver the drm_vblank_cleanup() function
warns us that the vblanks are still enabled. Fix that by calling
drm_crtc_vblank_off() in the malidp_unbind() function.

Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>