Claude review: drm/msm: Add basic perfcntr infrastructure

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

This adds `msm_perfcntr.c` with init/cleanup/resume/suspend stubs and group_idx/counter_base helpers.

**Issue: `msm_gpu_sysprof_no_perfcntr_zap()` NULL dereference.**

The inline function is changed to:

```c
return (refcount_read(&gpu->sysprof_active) > 1) ||
	READ_ONCE(gpu->perfcntrs->stream);
```

But `gpu->perfcntrs` is only set when `gpu->num_perfcntr_groups > 0` (which is a6xx+ only). For older GPUs (a2xx-a5xx), `gpu->perfcntrs` will be NULL, causing a NULL pointer dereference if this function is ever called on those GPUs. This should be guarded:

```c
return (refcount_read(&gpu->sysprof_active) > 1) ||
	(gpu->perfcntrs && READ_ONCE(gpu->perfcntrs->stream));
```

**Minor: `devm_kzalloc`/`devm_kfree` usage.** Using `devm_*` allocations then manually freeing in `msm_perfcntr_cleanup()` is a bit unusual. The `devm_kfree` calls in cleanup are necessary since the device outlives the cleanup, but mixing devm with manual lifetime management can be confusing. Not a bug, just worth noting.

**Minor: `msm_perfcntr_resume()` doesn't check `gpu->perfcntrs` for NULL** before accessing `gpu->perfcntrs->stream`, same concern as above.

---
Generated by Claude Code Patch Reviewer