drm/i915/icl: Add 10-bit support for hdmi

Starting Icelake silicon supports 10-bpc hdmi to support certain
media workloads. Currently hdmi supports 8 and 12 bpc. Plumbed
in support for 10 bit hdmi.

Cc: James Ausmus <james.ausmus@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180522002558.29262-18-paulo.r.zanoni@intel.com