drm/vc4: Use `vc4_perfmon_find()`

Similar to commit f2a4bcb2 ("drm/v3d: Use v3d_perfmon_find()"),
replace the open-coded `vc4_perfmon_find()` with the real thing.

Cc: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241004123817.890016-1-mcanal@igalia.com