drm/amdkfd: Fix variable set but not used warning

All warnings (new ones prefixed by >>):

   drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_svm.c: In function
'svm_range_deferred_list_work':
>> drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_svm.c:2067:22: warning:
variable 'p' set but not used [-Wunused-but-set-variable]
    2067 |  struct kfd_process *p;
         |

Fixes: 367c9b0f ("drm/amdkfd: Ensure mm remain valid in svm deferred_list work")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-By: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>