drm/vram-helper: switch over to the new pin interface

Stop using TTM_PL_FLAG_NO_EVICT.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/391603/?series=81973&rev=1