drm/i915: Add DSC support to MST path

Whenever we are not able to get enough timeslots
for required PBN, let's try to allocate those
using DSC, just same way as we do for SST.

v2: Removed intel_dp_mst_dsc_compute_config and refactored
    intel_dp_dsc_compute_config to support timeslots as a
    parameter(Ville Syrjälä)

v3: - Rebased
    - Added a debug to see that we at least try reserving
      VCPI slots using DSC, because currently its not visible
      from the logs, thus making debugging more tricky.
    - Moved timeslots to numerator, where it should be.

v4: - Call drm_dp_mst_atomic_check already during link
      config computation, because we need to know already
      by this moment if uncompressed amount of VCPI slots
      needed can fit, otherwise we need to use DSC.
      (thanks to Vinod Govindapillai for pointing this out)

v5: - Put pipe_config->bigjoiner_pipes back to original
      condition in intel_dp_dsc_compute_config
      (don't remember when I lost it)

v6: - Removed unnecessary drm_dp_mst_atomic_check as it is
      now always called in a newly introduced
      intel_dp_mst_find_vcpi_slots_for_bpp function
      (Vinod Govindapillai)

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221101094222.22091-5-stanislav.lisovskiy@intel.com