drm/i915: Use compressed bpp when calculating m/n value for DP MST DSC

For obvious reasons, we use compressed bpp instead of pipe bpp for
DSC DP SST case. Lets be consistent and use compressed bpp instead of
pipe bpp, also in DP MST DSC case.

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Fixes: d51f25eb ("drm/i915: Add DSC support to MST path")
Link: https://patchwork.freedesktop.org/patch/msgid/20230327064217.24033-1-stanislav.lisovskiy@intel.com