drm/i915/dp_mst: Fix symbol clock when calculating the DSC DPT bpp limit

The expected link symbol clock unit when calculating the DSC DPT bpp
limit is kSymbols/sec, aligning with the dotclock's kPixels/sec unit
based on the crtc clock. As opposed to this port_clock is used - which
has a 10 kbits/sec unit - with the resulting symbol clock in 10
kSymbols/sec units (disregarding the rounding error for the 13.5Gbps
rate). Fix the calculation using the expected 10x factor.

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/20240416221010.376865-3-imre.deak@intel.com