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>

Documentation/gpu/amdgpu.rst

@@ -52,6 +52,7 @@ AMDGPU Virtual Memory
    :doc: GPUVM
 
 .. kernel-doc:: drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
+   :internal:
 
 Interrupt Handling
 ------------------