drm/i915: remove force argument from disable_plane

The idea was good, but planes can have a fb even though
they're disabled. This makes the force argument useless
and always true, because only the commit function updates
state.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by(IVB): Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>