drm/i915/vlv: DPI FIFO empty check is not needed

While sending DPI SHUTDOWN command, we cannot wait for FIFO empty as
pipes are not disabled at that time. In case of MIPI we disable port
first and send SHUTDOWN command while pipe is still running and FIFOs
will not be empty, causing spurious error log

Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>