drm/amdgpu: Restore :internal: for amdgpu_vm.c documentation

This was accidentally dropped by the "drm/amdgpu: update documentation
for amdgpu_irq.c v3" change, resulting in the GPUVM documentation body
being included twice in the generated documentation.

Trivial.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>