drm/i915/dp: Inline do_dsc_compute_compressed_bpp()

With just the one platform independent loop left in
do_dsc_compute_compressed_bpp(), we don't really need the extra function
that is simply becoming increasingly hard to even figure out a decent
name for. Just merge the whole thing to
dsc_compute_compressed_bpp(). Good riddance to the short lived
do_dsc_compute_compressed_bpp().

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/91ae42cbdffe4938a665667955c577f887b92b9d.1738327620.git.jani.nikula@intel.com