Claude review: drm/msm: Add basic perfcntr infrastructure

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

Patch Review

Creates `msm_perfcntr.c` with init/cleanup/resume/suspend functions and the global state tracking structures.

**Bug -- NULL pointer dereference**: `msm_perfcntr_resume()` and `msm_perfcntr_suspend()` dereference `gpu->perfcntrs->stream` without checking if `gpu->perfcntrs` is NULL:
```c
int msm_perfcntr_resume(struct msm_gpu *gpu)
{
    guard(mutex)(&gpu->perfcntr_lock);
    return msm_perfcntr_resume_locked(gpu->perfcntrs->stream);
}
```
Since `gpu->perfcntrs` is only allocated when `num_perfcntr_groups > 0`, and these functions are called unconditionally from `adreno_runtime_resume()`/`adreno_runtime_suspend()` (and `adreno_recover()`), this will crash on any GPU that doesn't set up perfcntr tables (a2xx through a5xx).

**Bug -- same issue in `msm_gpu_sysprof_no_perfcntr_zap()`**:
```c
static inline bool
msm_gpu_sysprof_no_perfcntr_zap(struct msm_gpu *gpu)
{
    return (refcount_read(&gpu->sysprof_active) > 1) ||
        READ_ONCE(gpu->perfcntrs->stream);
}
```
Same NULL deref when `gpu->perfcntrs` is NULL. The fix for both is to add a `if (!gpu->perfcntrs)` early-return/guard.

**Nit**: `msm_perfcntr_cleanup()` uses `devm_kfree()` which is unusual -- since the memory was allocated with `devm_kzalloc()`, it will be freed automatically at device removal. The explicit `devm_kfree()` is fine for early cleanup, but mixing devm allocation with manual free patterns can be confusing. Consider using plain `kzalloc`/`kfree` since you're managing the lifetime manually anyway.

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Generated by Claude Code Patch Reviewer