drm/i915: Get the iboost setting based on the port type

Program the 'iboost_bit' based on what the VBT says it should be for
the specific port type, rather than assume it's always the same
for DP and HDMI.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468328376-6380-8-git-send-email-ville.syrjala@linux.intel.com