drm/i915/dsi: call wait_for_dsi_fifo_empty() for each dsi port

Add port parameter to wait_for_dsi_fifo_empty, and call it for each dsi
port.

We can now remove the transitional intel_dsi_pipe_to_port() function.

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>