drm/xlnx/zynqmp_dp: Fix function name zynqmp_dp_link_train() -> zynqmp_dp_train()

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

 drivers/gpu/drm/xlnx/zynqmp_dp.c:793: warning: expecting prototype for zynqmp_dp_link_train(). Prototype was for zynqmp_dp_train() instead

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