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

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

Refactor the logic to get the number of joined pipes. Start with a single
pipe and incrementally try additional pipes only if needed. While DSC
overhead is not yet computed here, this restructuring prepares the code to
support that in follow-up changes.

v2:
 - Remove fallback in case force-joiner configuration fails. (Imre)
 - Drop redundant MODE_OK assignment (Imre)
v3:
 - Align with the changes in intel_dp_mode_valid(). (Imre)
v4:
 - Set MODE_CLOCK_HIGH on DSC/rate failures aligning with SST case. (Imre)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patch.msgid.link/20260202103731.357416-9-ankit.k.nautiyal@intel.com