x86: video: Support copy framebuffer with probed devices

For PCI video devices that are not mentioned in the devicetree, U-Boot
does not bind a driver before relocation, since PCI is not fully probed
at that point. Furthermore it is possible for the video device to be on
a secondary bus which is not even scanned.

This is fine if the framebuffer is allocated in fixed memory, as it
normally is on x86. But when using this as a copy framebuffer, we also
need U-Boot to allocate its own cached framebuffer for working in. Since
the video driver is never bound before relocation, the framebuffer size
is never set and U-Boot does no allocation.

Add a new CONFIG option to reserve 16MB of memory for this eventuality.
This allows vesa devices to use the copy framebuffer.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Bin Meng <bmeng.cn@gmail.com>