drm/amdgpu: install ctx entities with cmpxchg

Since we removed the context lock we need to make sure that not two threads
are trying to install an entity at the same time.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 461fa7b0 ("drm/amdgpu: remove ctx->lock")
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>