drm/i915: Extract intel_ddi_get_buf_trans_hdmi()

Introduce intel_ddi_get_buf_trans_hdmi() and start using it where we
can.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171016145705.11780-3-ville.syrjala@linux.intel.com


Reviewed-by: James Ausmus <james.ausmus@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>