drm/i915/dp_mst: Rework pipe joiner logic in compute_config

drm/i915/dp_mst: Rework pipe joiner logic in compute_config

Similar to the DP SST, refactor mst_stream_compute_config() to iterate
over joiner candidates and select the minimal joiner configuration that
satisfies the mode requirements. This prepares the logic for future changes
that will consider DSC slice overhead.

v2:
 - Move the check for dotclock in the new helper and check for both DSC and
   non-DSC case. In case the check fails for non-DSC, fallback to DSC
   configuration. (Imre)
 - Propagate the return value from the core helper:
   mst_stream_compute_link_for_joined_pipes(). (Imre)
v3:
 - Use num_joined_pipes instead of num_pipes. (Imre)
v4:
 - Switch to 'ret == 0' for break condition to match SST code.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260202103731.357416-11-ankit.k.nautiyal@intel.com