i915: content-type property for HDMI connector

Added encoding of drm content_type property from drm_connector_state
within AVI infoframe in order to properly handle external HDMI TV
content-type setting.

This requires also manipulationg ITC bit, as stated in
HDMI spec.

v2:
 * Moved helper function which attaches content type property
   to the drm core, as was suggested.
   Removed redundant connector state initialization.

v3:
 * Removed caps in drm_content_type_enum_list.
   After some discussion it turned out that HDMI Spec 1.4
   was wrongly assuming that IT Content(itc) bit doesn't affect
   Content type states, however itc bit needs to be manupulated
   as well. In order to not expose additional property for itc,
   for sake of simplicity it was decided to bind those together
   in same "content type" property.

v4:
 * Added it_content checking in intel_digital_connector_atomic_check.
   Fixed documentation for new content type enum.

v5:
 * Moved patch revision's description to commit messages.

v6:
 * Minor naming fix for the content type enumeration string.

v7:
 * Fix parameter name for documentation and parameter alignment
   in order not to get warning. Added Content Type description to
   new HDMI connector properties section.

v8:
 * Thrown away unneeded numbers from HDMI content-type property
   description. Switch to strings desription instead of plain
   definitions.

v9:
 * Moved away hdmi specific content-type enum from
   drm_connector_state. Content type property should probably not
   be bound to any specific connector interface in
   drm_connector_state.
   Same probably should be done to hdmi_picture_aspect_ration enum
   which is also contained in drm_connector_state. Added special
   helper function to get derive hdmi specific relevant infoframe
   fields.

v10:
 * Added usage description to HDMI properties kernel doc.

v11:
 * Created centralized function for filling HDMI AVI infoframe, based
   on correspondent DRM property value.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180515135928.31092-3-stanislav.lisovskiy@intel.com
[vsyrjala: clean up checkpatch multiple blank lines warnings]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>