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

Currently, the number of joined pipes are determined early in the flow,
which limits flexibility for accounting DSC slice overhead. To address
this, recompute the joined pipe count during DSC configuration.

Refactor intel_dp_dsc_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:
 - Rename helper to intel_dp_compute_link_for_joined_pipes(). (Imre)
 - Move the check for max dotclock inside the helper so that if dotclock
   check fails for non DSC case for a given number of joined pipes, we
   are able to fallback to the DSC mode. (Imre)
v3:
 - Drop fallback to other joiner configurations, if the force joiner
   configuration fails. (Imre)
 - Check for maxdotclock limit for non-DSC case first and fall back to
   DSC if the check fails. (Imre)
 - Initialize ret to -EINVAL to handle case where we bail out early.
   (Imre)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260202103731.357416-6-ankit.k.nautiyal@intel.com

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