drm/msm/dsi: Appropriately set dsc->mux_word_size based on bpc

This field is currently unread but will come into effect when duplicated
code below is migrated to call drm_dsc_compute_rc_parameters(), which
uses the bpc-dependent value of the local variable mux_words_size in
much the same way.

The hardcoded constant seems to be a remnant from the `/* bpc 8 */`
comment right above, indicating that this group of field assignments is
applicable to bpc = 8 exclusively and should probably bail out on
different bpc values, until constants for other bpc values are added (or
the current ones are confirmed to be correct across multiple bpc's).

Fixes: b9080324

 ("drm/msm/dsi: add support for dsc data")
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/508943/
Link: https://lore.kernel.org/r/20221026182824.876933-6-marijn.suijten@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>