x86: Set up video framebuffer for coreboot before loading kernel

Currenlty we only set up video framebuffer when VIDEO_VESA driver is
used. With coreboot, VIDEO_COREBOOT driver is used instead. Since we
already saved VESA mode in the VIDEO_COREBOOT driver, now we can also
set up video framebuffer for coreboot before loading Linux kernel.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>