Use correct helper for getting max DSC bpc supported by the source.

Fixes: 1c56e9a3 ("drm/i915/dp: Get optimal link config to have best compressed bpp")
Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231213091632.431557-3-ankit.k.nautiyal@intel.com
(cherry picked from commit cd7b0b2d)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>