drm: add a check for x/y in drm_mode_setcrtc

legacy setcrtc ioctl does take a 32 bit value which might indeed
overflow

the checks of crtc_req->x > INT_MAX and crtc_req->y > INT_MAX aren't
needed any more with this

v2: -polish the annotation according to Daniel's comment

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Zhao Junwang <zhjwpku@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>