drm/i915: dsi: fix pipe-off timeout due to port vs. pipe disable ordering

If we disable first the port (by disabling DPI) and only then the
display pipe the pipe-off flag will never be set, possibly leading to a
hanged pipe state at the next modeset-enable.

Note that according to the VLV2 display cluster HAS, we should disable
the port before the pipe. This doesn't seem to match reality based on
the above and it's also asymmetric with the enabling sequence, where we
first enable the port and then the pipe.

v2:
- send the panel shutdown command before stopping the pipe, since this
  is the recommended sequence (Shobhit)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>