drm/i915/dp: Avoid forcing DSC BPC for MST case

For MST the bpc is hardcoded to 8, and pipe bpp to 24.
So avoid forcing DSC bpc for MST case.

v2: Warn and ignore the debug flag than to bail out. (Jani)

v3: Fix dbg message to mention forced bpc instead of bpp.

v4: Fix checkpatch longline warning.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230817142459.89764-9-ankit.k.nautiyal@intel.com