drm/i915: Fix HDMI 12bpc TRANSCONF bpc value

IBX BSpec says we must specify 8bpc in TRANSCONF for both 8bpc
and 12bpc HDMI output. Do so.

v2: Pass intel_crtc to intel_pipe_has_type()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chandra Konduru <Chandra.konduru@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>