Use signed location for drmModeSetPlane

DRM_IOCTL_MODE_SETPLANE crtc_x, crtc_y are s32.
This is to allow a destination location that is partially off screen.

Make this more obvious to users of libdrm by using signed crtc_x/_y
parameters for drmModeSetPlane() as well.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>