drm/v3d: Skip perfmon serialization while the device has no perfmon

Every job merges its finished fence into the per-queue accumulator so
that a job carrying a perfmon can later depend on everything still in
flight. This is a (relatively high) cost that every job pays before
it is submitted.

Perfmons only exist while userspace runs a performance query, so the
common case is a client paying that on every job for a dependency that is
never actually added, increasing the submission latency.

Address this situation by counting the number of perfmons alive on the
device and returning early while that count is zero. This means that if
no perfmon exists, v3d_serialize_for_perfmon() will bail out immediately.

Jobs submitted while no perfmon is alive stay unaccounted and may overlap
the first measured job. The count returns to zero whenever the last
perfmon is destroyed, so that window reopens on every measurement cycle.
Closing it would mean merging a fence on every submission for the
lifetime of the device, so it is a reasonable compromise for the average
use case.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patch.msgid.link/20260922-v3d-quick-exit-perfmon-serialize-v2-1-2a8021dc32f2@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>

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