drm/qxl: use iterator instead of dma_resv_shared_list

I'm not sure why it is useful to know the number of fences
in the reservation object, but we try to avoid exposing the
dma_resv_shared_list() function.

So use the iterator instead. If more information is desired
we could use dma_resv_describe() as well.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211129120659.1815-5-christian.koenig@amd.com