dm: video: Don't do anything in alloc_fb() when plat->size is zero

With DM VESA driver on x86 boards, plat->base/size/align are all
zeroes and starting address passed to alloc_fb() happens to be 1MB
aligned, so this routine does not trigger any issue. On QEMU with
U-Boot as coreboot payload, the starting address is within 1MB
range (eg: 0x7fb0000), thus causes failure in video_post_bind().

Actually if plat->size is zero, it makes no sense to do anything
in this routine. Add such check there.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>