drm/amdgpu: add module parameter for async_gfx_ring enablement

0 means disable async_gfx_ring and is the default setting
1 means enable async_gfx_ring

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Jack Xiao <jack.xiao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>