drm/i915/dp: Move max DSC BPP reduction one level higher

drm/i915/dp: Move max DSC BPP reduction one level higher

Now that {icl,xelpd}_dsc_compute_link_config() take .4 fixed point as
parameter, move the common max DSC BPP reduction one level higher. Use
intel_dp_dsc_bpp_step() to compute the step, and pass on to both
platform specific functions. (Though it's unused for now in
icl_dsc_compute_link_config()).

We can drop the pipe_bpp and connector parameters.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/62fa7f18ea49dce24c5d0ee7b2f0cbde9e2b609c.1738327620.git.jani.nikula@intel.com