drm/amdgpu: workaround to avoid SET_Q_MODE packets v2

It turned out that executing the SET_Q_MODE packet on every submission
creates to much overhead.

Implement a workaround which allows skipping the SET_Q_MODE packet if
subsequent submissions all use the same parameters.

v2: add a NULL check for ring_obj

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>