drm/i915/dp_mst: Fix DSC input BPP computation

drm/i915/dp_mst: Fix DSC input BPP computation

The branch or sink device decompressing a stream may have a limitation
on the input/uncompressed BPP, which is lower than the base line BPP
(determined by the sink's EDID). In some cases a stream with an input
BPP higher than this limit will be converted automatically by the device
decompressing the stream, by truncating the BPP, however in some cases
- seen at least in Dell dock's DP->HDMI converters - the decompression
will fail.

Fix the above by limiting the input BPP correctly. This is done already
correctly for SST outputs.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611153351.3013235-1-imre.deak@intel.com