drm/i915/dp_mst: Use helpers to get dsc min/max input bpc

Use helpers for source min/max input bpc with DSC.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241217093244.3938132-8-ankit.k.nautiyal@intel.com