x86: Move FRAMEBUFFER_SET_VESA_MODE etc to video Kconfig

CONFIG_FRAMEBUFFER_SET_VESA_MODE and CONFIG_FRAMEBUFFER_VESA_MODE
are not x86-specific, so move them to drivers/video/Kconfig and
make them depend on VIDEO_VESA driver. Some cosmetic fixes are
applied to the Kconfig help text as well.

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