drm: no-op out GET_STATS ioctl

Again only used by a tests in libdrm and by dristat. Nowadays we have
much better tracing tools to get detailed insights into what a drm
driver is doing. And for a simple "does it work" kind of question that
these stats could answer we have plenty of dmesg debug log spew.

So I don't see any use for this stat gathering complexity at all.

To be able to gradually drop things start with ripping out the
interfaces to it, here the ioctl.

To prevent dristat from eating its own stack garbage we can't use the
drm_noop ioctl though, since we need to clear the return data with a
memset.

Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>