sunxi: video: Dynamically reserve framebuffer memory

Only use CONFIG_SUNXI_MAX_FB_SIZE to reserve memory at the top when relocating
u-boot, and calculate the actual amount of memory necessary when setting up
the video-mode and use only that, freeing up some additional memory for use
by the kernel.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>