drm/i915: add config function for YCBCR420 outputs

This patch checks encoder level support for YCBCR420 outputs.
The logic goes as simple as this:
If the input mode is YCBCR420-only mode: prepare HDMI for
YCBCR420 output, else continue with RGB output mode.

It checks if the mode is YCBCR420 and source can support this
output then it marks the ycbcr_420 output indicator into crtc
state, for further staging in driver.

V2: Split the patch into two, kept helper functions in DRM layer.
V3: Changed the compute_config function based on new DRM API.
V4: Rebase
V5: Rebase
V6: Check and handle YCBCR420-only modes, discard the property
    based approach (Ville)
V7: Addressed review comments from Ville
    - add else case in 12BPC check.
    - extract ycbcr420 state inside hdmi_12bpc_possible function.
V8: Addressed review comments from Ville
    - Remove extra blank lines.
    - Remove "HDMI" from the description of ycbcr420 state variable.
    - Remove local variable, use crtc_state->ycbcr420 instead.
    Added r-b from Ville.
V9: Rebase
V10: Added r-b from Imre

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ander Conselvan de Oliveira <conselvan2@gmail.com>

Reviewed-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1500650709-14447-2-git-send-email-shashank.sharma@intel.com


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>