drm/i915/dp_mst: Track DSC enabled status on the MST link

drm/i915/dp_mst: Track DSC enabled status on the MST link

Track whether DSC is enabled on any CRTC on a link. On DP-SST (and DSI)
this will always match the CRTC's DSC state, those links having only a
single stream (aka CRTC). For instance, on DP-MST if DSC is enabled for
CRTC#0, but disabled for CRTC#1, the DSC/FEC state for these CRTCs will
be as follows:

CRTC#0:
 - compression_enable = true
 - compression_enabled_on_link = true
 - fec_enable = true for 8b10b, false for 128b132b

CRTC#1:
 - compression_enable = false
 - compression_enabled_on_link = true
 - fec_enable = true for 8b10b, false for 128b132b

This patch only sets compression_enabled_on_link for CRTC#0 above and
enables FEC on CRTC#0 if DSC was enabled on any other CRTC on the 8b10b
MST link. A follow-up change will make sure that the state of all the
CRTCs (CRTC#1 above) on an MST link is recomputed if DSC gets enabled on
any CRTC, setting compression_enabled_on_link and fec_enable for these.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://lore.kernel.org/r/20251015161934.262108-6-imre.deak@intel.com