drm/msm/dsi: Disallow 8 BPC DSC configuration for alternative BPC values

According to the `/* bpc 8 */` comment below only values for a
bits_per_component of 8 are currently hardcoded in place.  This is
further confirmed by downstream sources [1] containing different
constants for other BPC values (and different initial_offset too,
with an extra dependency on bits_per_pixel).  Prevent future mishaps by
explicitly disallowing any other bits_per_component value until the
right parameters are put in place and tested.

[1]: https://git.codelinaro.org/clo/la/platform/vendor/opensource/display-drivers/-/blob/DISPLAY.LA.2.0.r1-08000-WAIPIO.0/msm/sde_dsc_helper.c#L110-139

Fixes: b9080324

 ("drm/msm/dsi: add support for dsc data")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/508942/
Link: https://lore.kernel.org/r/20221026182824.876933-9-marijn.suijten@somainline.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>