drm/amdgpu: remove page flip work queue v3

Just use the system queue now that we don't block any more.

v2: handle DAL as well.
v3: agd: split DAL changes out

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Mykola Lysenko <mykola.lysenko@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com> (v1)