drm/amdgpu: remove excess function parameter

Fix below htmldocs build warning:

"warning: Excess function parameter 'vm_context' description in 'amdgpu_vm_init'"

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>