drm/amdgpu: Fix warnings while make xmldocs

This patch fixes following warnings.

./drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c:3011:
warning: Excess function parameter 'dev' description
in 'amdgpu_vm_get_task_info'

./drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c:3012:
warning: Function parameter or member 'adev' not
described in 'amdgpu_vm_get_task_info'

./drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c:3012:
warning: Excess function parameter 'dev' description
in 'amdgpu_vm_get_task_info'

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>