rk3288: Add VOP_MODE_NONE for headless devices

BUG=none
BRANCH=none
TEST=built w/ follow-up rialto patch

Change-Id: I166c75673c199e8c6860f601db6759cdc3cb9c96
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: ba0e0b639f12654ec54216d4366c688baa5b3253
Original-Change-Id: Id21c87ace2f4d381a1e374e1d5fe15cf1cd96da0
Original-Signed-off-by: David Hendricks <dhendrix@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/282047
Reviewed-on: http://review.coreboot.org/10779
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/src/soc/rockchip/rk3288/display.c b/src/soc/rockchip/rk3288/display.c
index ab297a2..d241e3e 100644
--- a/src/soc/rockchip/rk3288/display.c
+++ b/src/soc/rockchip/rk3288/display.c
@@ -54,6 +54,8 @@
 	mmu_config_range(lower / MiB, (upper - lower) / MiB, DCACHE_OFF);
 
 	switch (conf->vop_mode) {
+	case VOP_MODE_NONE:
+		return;
 	case VOP_MODE_AUTO_DETECT:
 		/* try EDP first, then HDMI */
 	case VOP_MODE_EDP: