drm: add helper functions for YCBCR420 handling

drm: add helper functions for YCBCR420 handling

This patch adds helper functions for YCBCR 420 handling.
These functions do:
- check if a given video mode is YCBCR 420 only mode.
- check if a given video mode is YCBCR 420 also mode.

V2: Added YCBCR functions as helpers in DRM layer, instead of
    keeping it in I915 layer.
V3: Added handling for YCBCR-420 only modes too.
V4: EXPORT_SYMBOL(drm_find_hdmi_output_type)
V5: Addressed review comments from Danvet:
    - %s/drm_find_hdmi_output_type/drm_display_info_hdmi_output_type
    - %s/drm_can_support_ycbcr_output/drm_display_supports_ycbcr_output
    - %s/drm_can_support_this_ycbcr_output/
		drm_display_supports_this_ycbcr_output
    - pass drm_display_info instead of drm_connector for consistency
    - For drm_get_highest_quality_ycbcr_supported doc, move the variable
      description above, and then the function description.
V6: Add only YCBCR420 helpers (Ville)
V7: Addressed review comments from Ville
    - Remove cea_vic_valid() check.
    - Fix indentation.
    - Make input parameters to helpers, const.

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Jose Abreu <Jose.Abreu@synopsys.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1499960000-9232-9-git-send-email-shashank.sharma@intel.com
[vsyrjala: Fix sparse indentation warn]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>