drm/i915/dp: Fix UHBR link M/N values

The link M/N ratio is the data rate / link symbol clock rate, fix things
up accordingly. On DP 1.4 this ratio was correct as the link symbol clock
rate in that case matched the link data rate (in bytes/sec units, the
symbol size being 8 bits), however it wasn't correct for UHBR rates
where the symbol size is 32 bits.

Kudos to Arun noticing in Bspec the incorrect use of link data rate in
the ratio's N value.

Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116131841.1588781-7-imre.deak@intel.com