drm/i915/mst: Convert intel_dp_mtp_tu_compute_config() to .4 format

Move towards always using the fxp q4 or .4 fixed point format for
compressed bpp. We'll need to pass the more accurate bpp to this
function later on.

Always use _x16 naming for variables that are in .4 fixed point for
clarity.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/887306a47ce4550226f5d54178f667a52840a11c.1738327620.git.jani.nikula@intel.com