drm/radeon: stop using dma_resv_excl_fence

Instead use the new dma_resv_get_singleton function.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220321135856.1331-8-christian.koenig@amd.com