drm/i915: Fix fastset vs. pfit on/off on HSW EDP transcoder

On HSW the pipe A panel fitter lives inside the display power well,
and the input MUX for the EDP transcoder needs to be configured
appropriately to route the data through the power well as needed.
Changing the MUX setting is not allowed while the pipe is active,
so we need to force a full modeset whenever we need to change it.

Currently we may end up doing a fastset which won't change the
MUX settings, but it will drop the power well reference, and that
kills the pipe.

Cc: stable@vger.kernel.org
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: d19f958d

 ("drm/i915: Enable fastset for non-boot modesets.")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190425162906.5242-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>