drm/i915: Only set CURSOR_PIPE_CSC_ENABLE when cursor is enabled

It seems cleaner if we keep CURCNTR at 0 when the cursor is disabled,
so don't set the CURSOR_PIPE_CSC_ENABLE bit unless the cursor is
enabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>