Patchset ported, tested, and sent to kernel-team mailing list

** Description changed:

  SRU Justification:
  
  Impact:
  On some setups, while the monitor and the GPU support display modes with 
pixel clocks of up to 600MHz, the connector might not. This prevents RGB 
encoding for 4k60Hz, but YCbCr420 encoding might still be possible. However, 
which color mode is used is decided before the pixel clock capabilities are 
checked, causing the check to fail and discarding 4k60Hz from the list of 
possible display modes.
  
  Fix:
  This patch fixes the problem by retrying to find a display mode with YCbCr420 
enforced and using it, if it is valid. It's very similar to a patch submitted 
to amdgpu which fixed the same problem.
  
  Testcase:
- Find a PC with a current Intel iGPU, but only a hdmi 1.4 output. Connect a 
4k@60Hz display supporting YCbCr420 encoding to the hdmi port. Without the 
patch the maximum that can be set via xrandr is 3840 × 2160 30Hz. With the 
Patch 3840 × 2160 60Hz can be selected which will use YCbCr420 automatically.
+ I personally tested on a Clevo NV40MB, but generally: Find a PC with a 
current Intel iGPU, but only a HDMI 1.4 output. Connect a 4k@60Hz display 
supporting YCbCr420 encoding to the HDMI port. Without the patch the maximum 
that can be set via xrandr is 3840 × 2160 30Hz. With the Patch 3840 × 2160 60Hz 
can be selected which will use YCbCr420 automatically.
  
- Prerequistie:
+ Prerequisite (included in this email patchset as 1/4):
  
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3c4442aa22878091f16c8d9592f5f5b6a94d1556
  
  Patchset already got accepted upstream and reached the torvalds tree:
  
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=eacba74d4d561ea6487d944417526e1b025cbebd
  
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=84d95f77f4aea3f22a486cd04777afd4ab0f0ea5
  
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=388b863509f76f6a5ecedd7ffdaf184aa813241e
  and needs only a minor modifications to apply to ubuntu-focal/hwe-5.8
  
  Commit-hashes:
  3c4442aa22878091f16c8d9592f5f5b6a94d1556
  eacba74d4d561ea6487d944417526e1b025cbebd
  84d95f77f4aea3f22a486cd04777afd4ab0f0ea5
  388b863509f76f6a5ecedd7ffdaf184aa813241e

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https://bugs.launchpad.net/bugs/1934489

Title:
  Make Intel GPUs choose YCbCr420 encoding automatically when required
  for 4k 60Hz output

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