drm/vc4: plane: Fix incorrect handling of GEN_6_D in vc4_plane_async_… #3492

Job	Run time
build (bcm2835, arm, bcm2835_defconfig, kernel)	10m 19s
build (arm64, arm64, defconfig, kernel8)	39m 1s
build (bcmrpi, arm, bcmrpi_defconfig, kernel)	31m 9s
build (bcm2709, arm, bcm2709_defconfig, kernel7)	32m 0s
build (bcm2711, arm, bcm2711_defconfig, kernel7l)	33m 30s
build (bcm2711_arm64, arm64, bcm2711_defconfig, kernel8)	42m 44s
build (bcm2711_rt, arm64, bcm2711_rt_defconfig, kernel8_rt)	42m 36s
build (bcm2712, arm64, bcm2712_defconfig, kernel_2712)	42m 34s
	4h 33m 53s

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