video: Correctly handle multiple framebuffers

At present video_bottom is set to the bottom of each framebuffer when it
is allocated. This is not correct, since it should hold the bottom of the
entire area available for framebuffers.

Fix this by adding a private address in the uclass which keeps track of
the next available spot for a framebuffer.

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