drm/i915/dp: Prefer DSC over 6 bpc uncompressed output for HDMI sinks

For modes which fit through the link uncompressed only with a 6 bpc
pipe BPP, the link config currently selects 6 bpc with dithering even
if the sink supports DSC.

For HDMI sinks behind a DP to HDMI protocol converter this is the
wrong preference: HDMI knows no 6 bpc transport format, so the
converter has to expand the dithered 6 bpc stream back to 8 bpc for
the HDMI link anyway, and DSC with an at least 8 bpc input provides
a better output quality than that.
Prefer DSC, following the pattern commit ba49a4643cf5 ("drm/i915/dp:
Set min_bpp limit to 30 in HDR mode") uses for HDR: keep the
uncompressed minimum pipe BPP at 8 bpc if the sink supports DSC,
making the uncompressed link config fail for such modes and the mode
fall back to DSC.
As there, if the DSC computation fails, the mode falls back to YCbCr
4:2:0 where supported, or gets rejected.

Besides the output quality, some converters can't even display a
6 bpc stream at high pixel clocks.
Synaptics VMM PCON based DP to HDMI 2.1 adapters from two device
families (branch device IDs SYNAq and SYNAa) output corrupted FRL
timings for an uncompressed RGB 6 bpc 4k120 (1188 MHz) stream,
resulting in a black screen, while the same mode works with DSC
(12 bpc input) and 6 bpc works at lower pixel clocks.
Windows and macOS drive 4k120 on these devices only via DSC.

A lower bpc limit explicitly requested via the max bpc connector
property is still honored.
This keeps the current uAPI behavior (exercised by IGT kms_dither)
and provides an escape hatch for sinks with a broken DSC
implementation.
DP and eDP sinks, and HDMI sinks without DSC support, are not
affected and keep falling back to 6 bpc.

Tested on PTL (xe) with the above PCONs and an LG OLED G4.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Alexander Kaplan <alexander.kaplan@sms-medipool.de>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://patch.msgid.link/20260718105207.5565-2-alexander.kaplan@sms-medipool.de

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