rockchip: Don't enable AFBC if width > 2560

rockchip: Don't enable AFBC if width > 2560

BUG=chrome-os-partner:62585
TEST=Plug in monitor with native resolution over 2560 (eg 4k monitor)
	and verify the output looks correct.

Change-Id: Iec925f54d74be19342d710313e5ac4ddca529e62
Signed-off-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-on: https://chromium-review.googlesource.com/438831
Commit-Ready: Douglas Anderson <dianders@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>