drm/amdgpu: shrink critical section in amdgpu_amdkfd_gpuvm_free_memory_of_gpu

Reduce the mem->lock`s protected code area, no need to protect pr_debug.
This also simplifies error handling.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>