rk3288: Add VOP_MODE_NONE for headless devices

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

Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/282047
Change-Id: Id21c87ace2f4d381a1e374e1d5fe15cf1cd96da0
Reviewed-on: https://chromium-review.googlesource.com/284098

src/soc/rockchip/rk3288/display.c

@@ -54,6 +54,8 @@ void rk_display_init(device_t dev, u32 lcdbase,
 	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:

src/soc/rockchip/rk3288/include/soc/vop.h

@@ -107,6 +107,7 @@ enum vop_modes {
 	 */
 	VOP_MODE_EDP = 0,
 	VOP_MODE_HDMI,
+	VOP_MODE_NONE,
 	VOP_MODE_AUTO_DETECT,
 	VOP_MODE_UNKNOWN,
 };