drm/connector: hdmi: Prefer YUV422 over dropping RGB to 8 bpc

[popcornmix]

When userspace requests more than 8 bpc (e.g. Kodi sets max_bpc=12) and the mode only fits the TMDS limit with RGB at 8 bpc, the HDMI state helper picks 8-bit RGB. YUV422 at 12 bpc needs the same TMDS rate as RGB at 8 bpc, but it was never tried because RGB was tried at every bpc before YUV422.

This changes the order to:

1. RGB from max_bpc down to 10 bpc
2. YUV422 from max_bpc down to 8 bpc
3. RGB at 8 bpc
4. YUV420 (unchanged)

With max_bpc=8 the behaviour is unchanged. RGB at 10 bpc is still preferred over YUV422 at 12 bpc, matching drm_test_check_max_tmds_rate_bpc_fallback_ignore_yuv422.

Tested on Pi 5 with Kodi playing a 4K 59.94 Hz 10-bit HDR clip on a TV that supports YCbCr 4:2:2:

• before: 3840x2160@59.94, RGB 8 bpc, 593.4 MHz
• after: 3840x2160@59.94, YUV 4:2:2 12 bpc, 593.4 MHz; the AVI infoframe signals YCbCr 4:2:2

The HDMI state helper KUnit tests give the same 8 failures before and after this change. They fail on rpi-6.18.y because max_bpc now starts at 8, so those tests never request more than 8 bpc.

[6by9]

That should do what we want with as small a change to upstream code as possible.