drm/i915: Clamp cursor coordinates to int16_t range

We store cursor_x/y as int16_t internally, but the user provided
coordinates are int32_t. Clamp the coordinates so that they don't
overflow the int16_t. Since the cursor is only 64x64 in size, the
clamping can't cause any visual changes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>