drm/amdgpu: Fix __rcu fence pointer accesses

drm/amdgpu: Fix __rcu fence pointer accesses

Building for ARCH=um with W=1 C=1 makes sparse report "incompatible
types in comparison expression (different address spaces)" warnings in
the KFD code, exposed after UML builds were enabled:

- amdgpu_amdkfd_fence.c compares the __rcu-annotated dma_fence.ops
  pointer directly in to_amdgpu_amdkfd_fence().
- amdgpu_amdkfd_gpuvm.c compares the __rcu eviction fence pointer
  directly in amdgpu_amdkfd_gpuvm_restore_process_bos().

Fixes: af3f2f5db265 ("drm/amdgpu: Remove UML build exclusion from Kconfig")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202607091659.SHEscT0c-lkp@intel.com/
Cc: Harry Wentland <harry.wentland@amd.com>
Assisted-by: Copilot:Claude-Opus-4.8
Signed-off-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 764f241ad227bb942e5b0b8b4d9898f1a4175605)