drm/i915/dsi: move wait_for_dsi_fifo_empty to intel_dsi.c

wait_for_dsi_fifo_empty can be static in intel_dsi.c. No functional
changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-By: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>