drm/i915/bios: split up iboost to hdmi and dp bitfields

This is according to the style all over the place. No functional
changes.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/cf5f0e5d10851db3796e632836d23551f59cc412.1503670197.git.jani.nikula@intel.com