drm/i915/dp_mst: Sanitize calculating the DSC DPT bpp limit

Instead of checking each compressed bpp value against the maximum
DSC/DPT bpp, simplify things by calculating the maximum bpp upfront and
limiting the range of bpps looped over using this maximum.

While at it add a comment about the origin of the DSC/DPT bpp limit.

Bspec: 49259, 68912

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240416221010.376865-7-imre.deak@intel.com