drm/i915/dp_mst: Use output_format to get the final link bpp

The final link bpp used to calculate the m_n values depend on the
output_format. Though the output_format is set to RGB for MST case and
the link bpp will be same as the pipe bpp, for the sake of semantics,
lets calculate the m_n values with the link bpp, instead of pipe_bpp.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230817142459.89764-4-ankit.k.nautiyal@intel.com