drm/xlnx/zynqmp_disp: Fix incorrectly named enum 'zynqmp_disp_layer_id'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/xlnx/zynqmp_disp.c:101: warning: expecting prototype for enum zynqmp_disp_id. Prototype was for enum zynqmp_disp_layer_id instead

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>