drm/i915/dp_mst: Use the correct connector while computing the link BPP limit on MST

drm/i915/dp_mst: Use the correct connector while computing the link BPP limit on MST

Atm, on an MST link in DSC mode
intel_dp_compute_config_link_bpp_limits() calculates the maximum link
bpp limit using the MST root connector's DSC capabilities. That's not
correct in general: the decompression could be performed by a branch
device downstream of the root branch device or the sink itself.

Fix the above by passing to intel_dp_compute_config_link_bpp_limits()
the actual connector being modeset, containing the correct DSC
capabilities.

Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Fixes: 1c5b72daff46 ("drm/i915/dp: Set the DSC link limits in intel_dp_compute_config_link_bpp_limits")
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250509180340.554867-2-imre.deak@intel.com
(cherry picked from commit 266e2fcfe2ea0d062ea392cd22f6250ae0d11c04)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>