drm/amdgpu: add option for force enable multipipe policy for compute

Useful for testing the effects of multipipe compute without recompiling.

Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>