drm/v3d: Refactor perfmon locking

v3d exposes a single set of performance counters per core, so at any
moment at most one performance monitor can be programmed in HW. In
software, this singleton is represented by v3d_dev->active_perfmon, but
until now nothing actually serialized access to it: scheduler callbacks,
the GPU-reset path, and perfmon ioctls all read and wrote that field
lock-free.

The existence of v3d_perfmon->lock mutex did not close the gap. It
serialized start/stop of *one* perfmon object against itself, but the
invariant that needs protection is device-wide: there can be exactly one
active perfmon at any moment in HW. Two threads acting on different
perfmon objects could race through v3d_dev->active_perfmon and the
counter registers, leaving software and HW out of sync.

This commit moves the locking to where the invariant actually lives. Group
the active perfmon pointer with a device-wide spinlock and route every
state transition (job start, job completion, set global, reset,
suspend/resume, destruction) through a small set of locked entry points
that are the only mutators of the HW counters.

Some design improvements needed to be made for the refactor:

  1. Stop the perfmon from the IRQ handler at job-completion time (the
     natural boundary for "active perfmon follows the active job"). This
     required a change from a mutex to a spinlock. This solves another
     issue of the existing design: perfmon start/stop was exclusively
     attached to run_job() callbacks, which means that if nothing was
     further queued up, a perfmon would never actually be stopped.

  2. Pause/resume the HW counters across runtime-PM transitions without
     dropping the software reference. This preserves the perfmon state
     while the device is idle.

  3. Move the global perfmon lifecycle management to the set_global
     IOCTL. This simplifies the logic in v3d_perfmon_start() and
     v3d_perfmon_stop(), as there is no need to always check if the
     global perfmon is enabled.

  4. v3d_perfmon_get_values_ioctl() doesn't stop the perfmon when
     capturing the values. All lifecycle management is handled by the
     job (for per-job perfmons) or the set_global IOCTL (for global
     perfmons).

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patch.msgid.link/20260706-v3d-perfmon-lifetime-v4-1-d7b312ff2c83@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>

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