drm/i915/dp: Separate out function to get compressed bpp with joiner

Pull the code to get joiner constraints on maximum compressed bpp into
separate function.

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/20230817142459.89764-16-ankit.k.nautiyal@intel.com