fbdev: ignore VESA modes if framebuffer does not support them

Currently, it is possible to set a graphics VESA mode at boot time via the
vga= parameter even when no framebuffer driver supporting this is
configured.  This could lead to the system booting with a black screen,
without a usable console.

Fix this problem by only allowing to set graphics modes at boot time if a
supporting framebuffer driver is configured.

Signed-off-by: Michal Januszewski <spock@gentoo.org>
Acked-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>