drm/amdgpu: More accurate description of a function param

Add more accurate description of the pe parameter of function
amdgpu_vm_sdma_udpate and amdgpu_vm_cpu_update

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Reviewed-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>