drm/amdgpu: cleanup amdgpu_vm_init()

Currently only way to create compute vm is through
amdgpu_vm_make_compute(). So vm_context isn't required
anymore for amdgpu_vm_init().

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