drm/i915/dp_mst: Remove stream count assert from intel_dp_check_mst_status()

drm/i915/dp_mst: Remove stream count assert from intel_dp_check_mst_status()

There doesn't seem to be a reason to assert for a non-negative stream
counter in intel_dp_check_mst_status() in particular, remove it. There
is now an equivalent assert in intel_dp_mst_dec_active_streams().

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250404150310.1156696-4-imre.deak@intel.com