drm/amdgpu: mitigate workaround for i915

To be able to use DRI_PRIME with amdgpu and i915 we add all our fences
only as exclusive ones.

Disable that behavior when sharing between amdgpu itself cause it
hinders concurrent execution.

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>