drm/i915/mst: change return value of mst_stream_find_vcpi_slots_for_bpp()

The callers of mst_stream_find_vcpi_slots_for_bpp() don't need the
returned slots for anything. On the contrary, they need to jump through
hoops to just distinguish between success and failure. Just return 0
instead of slots from mst_stream_find_vcpi_slots_for_bpp() for success,
and simplify the callers.

There's a pointless ret local variable that we can drop in the process.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7b3671a548c893b1bb62151d41f90bb8ce842ccc.1735912293.git.jani.nikula@intel.com