drm/i915: Replace dig_port->port with encoder port for BXT DPLL selection

Replace dig_port->port with encoder->port in the BXT DPLL selection.
We can do this because both the master encoder and the fake MST encoders
have the same encoder->port value, whereas using dig_port->port only
worked for the master encoder since the fake encoders were't derived
from intel_digital_port. This eliminates the DP MST special case.

Do this by hand because spatch is having problems with the control
flow due to the dig_port assignment happening in two different
branches.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171031205123.13123-8-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>