drm/amdgpu: block CE CS if not explicitely allowed by module option

The Constant Engine found on gfx6-gfx10 HW has been a notorious source of
problems.

RADV never used it in the first place, radeonsi only used it for a few
releases around 2017 for gfx6-gfx9 before dropping support for it as
well.

While investigating another problem I just recently found that submitting
to the CE seems to be completely broken on gfx9 for quite a while.

Since nobody complained about that problem it most likely means that
nobody is using any of the affected radeonsi versions on current Linux
kernels any more.

So to potentially phase out the support for the CE and eliminate another
source of problems block submitting CE IBs unless it is enabled again
using a debug flag.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>