drm/i915/dp_mst: Fix BW limit check when calculating DSC DPT bpp

The DSC DPT bpp limit check should only fail if the available DPT BW is
less than the required BW, fix the check accordingly.

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/20240416221010.376865-4-imre.deak@intel.com