drm/amdgpu: Fix NULL pointer dereference in VRAM logic for APU devices

drm/amdgpu: Fix NULL pointer dereference in VRAM logic for APU devices

Previously, APU platforms (and other scenarios with uninitialized VRAM managers)
triggered a NULL pointer dereference in `ttm_resource_manager_usage()`. The root
cause is not that the `struct ttm_resource_manager *man` pointer itself is NULL,
but that `man->bdev` (the backing device pointer within the manager) remains
uninitialized (NULL) on APUs—since APUs lack dedicated VRAM and do not fully
set up VRAM manager structures. When `ttm_resource_manager_usage()` attempts to
acquire `man->bdev->lru_lock`, it dereferences the NULL `man->bdev`, leading to
a kernel OOPS.

1. **amdgpu_cs.c**: Extend the existing bandwidth control check in
   `amdgpu_cs_get_threshold_for_moves()` to include a check for
   `ttm_resource_manager_used()`. If the manager is not used (uninitialized
   `bdev`), return 0 for migration thresholds immediately—skipping VRAM-specific
   logic that would trigger the NULL dereference.

2. **amdgpu_kms.c**: Update the `AMDGPU_INFO_VRAM_USAGE` ioctl and memory info
   reporting to use a conditional: if the manager is used, return the real VRAM
   usage; otherwise, return 0. This avoids accessing `man->bdev` when it is
   NULL.

3. **amdgpu_virt.c**: Modify the vf2pf (virtual function to physical function)
   data write path. Use `ttm_resource_manager_used()` to check validity: if the
   manager is usable, calculate `fb_usage` from VRAM usage; otherwise, set
   `fb_usage` to 0 (APUs have no discrete framebuffer to report).

This approach is more robust than APU-specific checks because it:
- Works for all scenarios where the VRAM manager is uninitialized (not just APUs),
- Aligns with TTM's design by using its native helper function,
- Preserves correct behavior for discrete GPUs (which have fully initialized
  `man->bdev` and pass the `ttm_resource_manager_used()` check).

v4: use ttm_resource_manager_used(&adev->mman.vram_mgr.manager) instead of checking the adev->gmc.is_app_apu flag (Christian)

Reviewed-by: Christian König <christian.koenig@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>