drm/i915/dp_mst: Calculate the BW overhead in intel_dp_mst_find_vcpi_slots_for_bpp()

The next patch will calculate the PBN value directly from the pixel data
rate and the BW allocation overhead, not requiring the data, link M/N
and TU values for this. To prepare for that move the calculation of BW
overheads from intel_dp_mst_compute_m_n() to
intel_dp_mst_find_vcpi_slots_for_bpp().

While at it store link_bpp in a .4 fixed point format.

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/20231116131841.1588781-8-imre.deak@intel.com