drm/i915: Fix cursor visibility check with negative coordinates

When the cursor x coordinate is exactly -cursor_width, the cursor is
invisible. And obviously the same holds for the y coordinate and
cursor_height.

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