drm/vmwgfx: Always evict vram _before_ disabling it

Other way round is a bit inconsistent (but not buggy in any kind).
This is prep work so that ttm_resource_manager_set_used can assert
that the resource manager is empty.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Cc: VMware Graphics <linux-graphics-maintainer@vmware.com>
Cc: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201211162942.3399050-2-daniel.vetter@ffwll.ch