[PATCH] drm/i915/display Try YCbCr420 color when RGB fails

[Werner Sembach]

When encoder validation of a display mode fails, retry with less bandwidth
heavy YCbCr420 color mode, if available. This enables some HDMI 1.4 setups
to support 4k60Hz output, which previously failed silently.

AMDGPU had nearly the exact same issue. This problem description is
therefore copied from my commit message of the AMDGPU patch.

On some setups, while the monitor and the gpu support display modes with
pixel clocks of up to 600MHz, the link encoder might not. This prevents
YCbCr444 and RGB encoding for 4k60Hz, but YCbCr420 encoding might still be
possible. However, which color mode is used is decided before the link
encoder capabilities are checked. This patch fixes the problem by retrying
to find a display mode with YCbCr420 enforced and using it, if it is
valid.

I'm not entierly sure if the second
"if (HAS_PCH_SPLIT(dev_priv) && !HAS_DDI(dev_priv))" check in
intel_hdmi_compute_config(...) after forcing ycbcr420 is necessary. I
included it to better be safe then sorry.

Signed-off-by: Werner Sembach <wse@xxxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
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Rebased from 5.12 to drm-tip and resend to resolve merge conflict.