drm/amdgpu: cleanup get_invalidate_req v2

The two hubs are just instances of the same hardware,
so the register bits are identical.

v2: keep the function pointer

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