drm/qxl: fix usage of ttm_bo_init

We need to use ttm_bo_init_reserved here to make sure
that the BO is pinned before it becomes visible on the LRU.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/392561/?series=82199&rev=1