drivers: video: Clear framebuffer when reserving

This fixes an issue where in some situations, the fixed framebuffer is
re-used (understandably), and with a warm reset is shown again briefly
during video initialization, until U-Boot renders something new.

This can be observed on the Pinebook Pro (RK3399, eDP), once the reset
issue of the panels are worked around correctly.

Signed-off-by: Samuel Dionne-Riel <samuel@dionne-riel.com>

Author: samueldr

drivers/video/video-uclass.c

@@ -113,6 +113,9 @@ static ulong alloc_fb(struct udevice *dev, ulong *addrp)
 	size = alloc_fb_(plat->align, plat->size, addrp);
 	plat->base = *addrp;
 
+	/* Clear the reserved famebuffer; some data might be sticking in from warm reset. */
+	memset(plat->base, 0x00, size);
+
 	return size;
 }