drm/i915/mst: remove crtc_state->pbn

The crtc_state->pbn member is only used as a temporary variable within
mst_stream_find_vcpi_slots_for_bpp(). Remove it as unnecessary.

Suggested-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/726aaadbd425057dfd854e42417bcf8d69b769d3.1735912293.git.jani.nikula@intel.com