drm/amdgpu: fix pipeline sync v2

This fixes a potential memory leak of dma_fence objects in the CS code
as well as glitches in firefox because of missing pipeline sync.

v2: use the scheduler instead of the fence context

Signed-off-by: Christian König <christian.koenig@amd.com>
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/2323
Tested-by: Michal Kubecek <mkubecek@suse.cz>
Tested-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230109130120.73389-1-christian.koenig@amd.com