drm/i915/dp_mst: Align TUs to avoid splitting symbols across MTPs

drm/i915/dp_mst: Align TUs to avoid splitting symbols across MTPs

Symbols consisting of multiple (4) TU timeslots may get split across
MTPs when using 2 or 1 link lanes. Avoid this, as required by Bspec by
aligning the allocated TUs to 2 when using 2 lanes and 4 when using 1
lane.

Atm, we also have to align the PBNs used to allocate BW along the MST
path, since DRM core keeps track of its own TU value, derived from the
PBN and that TU value must match what the driver calculates.

On some platforms the alignment is only required on 8b/10b links, a
follow-up patch will remove the limitation for those.

Bspec: 49266, 68922

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610164933.2947366-2-imre.deak@intel.com