drm/i915: Don't use HDMI 12bpc when cloning with other encoder types

When cloning HDMI with other output types, we can't use 12bpc since the
clocks for the other encoder types would be off. So have
intel_hdmi_compute_config() check if there are other encoders besides
HDMI being fed from the same pipe, and if so, pick 8bpc insted if 12bpc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>