drm/i915: prepare pipe for YCBCR420 output

To get HDMI YCBCR420 output, the PIPEMISC register should be
programmed to:
- Generate YCBCR output (bit 11)
- In case of YCBCR420 outputs, it should be programmed in full
  blend mode to use the scaler in 5x3 ratio (bits 26 and 27)

This patch:
- Adds definition of these bits.
- Programs PIPEMISC for YCBCR420 outputs.
- Adds readouts to compare HW and SW states.

V2: rebase
V3: rebase
V4: rebase
V5: added r-b from Ander
V6: Handle only YCBCR420 outputs (ville)
V7: rebase
V8: Addressed review comments from Ville
    - Add readouts for state->ycbcr420 and 420 pixel_clock.
    - Handle warning due to mismatch in clock for ycbcr420 clock.
    - Rename PIPEMISC macros to match the Bspec.
    - Add a debug print stating if YCBCR 4:2:0 output enabled.
    Added r-b from Ville
V9: Addressed review comments from Imre:
    - Add 420 mode clock adjustment in intel_hdmi_mode_valid to
      prevent 420_only modes getting rejected for high clock.
    - Add port clock adjustment for ycbcr420 modes in ddi_get_clock
    - Rename macros as per Ville's suggestion.
    - Remove unnecessary wl changes.
V10: Added r-b from Imre
V11: Fixed faulty dotclock handling, and addressed missing comment
     from previous set of review comments (Imre)
V12: Fixed dotclock for 12bpc too, removed 420 check for GEN < 10

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

Reviewed-by: 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/1500904172-31717-1-git-send-email-shashank.sharma@intel.com


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