drm: Add drm_plane_force_disable()

drm_plane_force_disable() will forcibly disable the plane even if user
had previously requested the plane to be enabled.

This can be used to force planes to be off when restoring the fbdev
mode.

The code was simply pulled from drm_framebuffer_remove(), which now
calls the new function as well.

v2: Check plane->fb in drm_plane_force_disable(), drop bogus comment
    about disabling crtc

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>