drm/i915/dp_mst: Add support for fractional compressed link bpps on MST

drm/i915/dp_mst: Add support for fractional compressed link bpps on MST

Add support for a fractional compressed link bpp on an MST link. Leave
the actual enabling of fractional bpps to a follow-up change.

While at it add an assert before the bpp loop, that the min and max bpps
are aligned to the bpp step. This should hold regardless of the non-DSC/DSC
or MST/UHBR-SST modes.

This keeps the mode validation and DSC->DPT BW specific maximum link
bpps as rounded-down integer values still, changing those to a
fractional value is left for later, add here TODO comments for them.

v2:
- Align the min/max bpp value to the bpp step.
- Assert that the min/max bpp values are aligned to the bpp step.

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250509180340.554867-9-imre.deak@intel.com