encoder->type isn't reliable for DP/HDMI encoders, so pass the type
explicity to skl_set_iboost(). Also take the opportunity to streamline
the code.

v2: Clean up the argument types to skl_ddi_set_iboost() while at it
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171016145705.11780-4-ville.syrjala@linux.intel.comReviewed-by: James Ausmus <james.ausmus@intel.com>