drm/i915: prepare scaler for YCBCR420 modeset

To get a YCBCR420 output from intel platforms, we need one
scaler to scale down YCBCR444 samples to YCBCR420 samples.

This patch:
- Does scaler allocation for HDMI ycbcr420 outputs.
- Programs PIPE_MISC register for ycbcr420 output.

V2: rebase
V3: rebase
V4: rebase
V5: addressed review comments from Ander:
    - No need to check both scaler_user && hdmi_output.
      Check for scaler_user is enough.
V6: rebase
V7: Do not create a new scaler user, use existing pipe scaler user.
V8: rebase
V9: Addressed review comments from Ville:
    - Remove leftover comment for HDMI scaler user.
    - Remove unnecessary blank line.
    - Make scaler alocation failure a DEBUG log instead of ERROR.
    Added r-b from Ville
V10: Update commit message as per latest code (Imre)
     Added r-b from Imre

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Ander Conselvan De Oliveira <conselvan2@gmail.com>
Cc: Imre Deak <imre.deak@intel.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/1500650709-14447-3-git-send-email-shashank.sharma@intel.com


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