LSPCON chips can generate YCBCR outputs, if asked nicely :).

In order to generate YCBCR 4:2:0 outputs, a source must:
- send YCBCR 4:4:4 signals to LSPCON
- program color space as 4:2:0 in AVI infoframes

Whereas for YCBCR 4:4:4 outputs, the source must:
- send YCBCR 4:4:4 signals to LSPCON
- program color space as 4:4:4 in AVI infoframes

So for both 4:2:0 as well as 4:4:4 outputs, we are driving the
pipe for YCBCR 4:4:4 output, but AVI infoframe's color space
information indicates LSPCON FW to start scaling down from YCBCR
4:4:4 and generate YCBCR 4:2:0 output. As the scaling is done by
LSPCON device, we need not to reserve a scaler for 4:2:0 outputs.

V2: rebase
V3: Addressed review comments from Ville
    - add enum crtc_output_format instead of bool ycbcr420
    - use crtc_output_format=4:4:4 for modeset of LSPCON 4:2:0 output
      cases in this way we will have YCBCR 4:4:4 framework ready (except
      the ABI part)
V4: Added r-b from Maarten (for v3)
    Addressed review comments from Ville:
    - Do not add a non-atomic state variable to determine lspcon output.
      Instead add bool in CRTC state to indicate lspcon based scaling.
V5: Addressed review comments from Ville:
    - Change the state bool name from external scaling to something more
      relavent.
    - Keep the info and adjusted_mode structures const.
    - use crtc_state instead of pipe_config.
    - Push all the config change into lspcon_ycbcr420_config function.
V6: Rebase, small changes to accommodate changes in patch 2.
V7: Fixed checkpatch warnings for alignment
V8: Rebase

    PS: Ignored following warnings to match the current formatting:
    drm/i915: Add YCBCR 4:2:0/4:4:4 support for LSPCON
     -:53: CHECK:SPACING: spaces preferred around that '<<' (ctx:VxV)
    #53: FILE: drivers/gpu/drm/i915/i915_reg.h:8721:
    +#define  TRANS_MSA_SAMPLING_444        (2<<1)
                                          ^
    -:54: CHECK:SPACING: spaces preferred around that '<<' (ctx:VxV)
    #54: FILE: drivers/gpu/drm/i915/i915_reg.h:8722:
    +#define  TRANS_MSA_CLRSP_YCBCR         (2<<3)
V9: Rebase
V10: Rebase
V11: Rebase

Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1539325394-20788-8-git-send-email-shashank.sharma@intel.com