fbdev: do not allow VESA modes without compiled-in drivers

Do not accept VESA modes by the "vga=" kernel parameter if there is no
frame buffer driver compiled-in to handle it.

Also, there is a comment added to the Kconfig description after Werner
Lemberg's suggestion

Addresses http://bugzilla.kernel.org/show_bug.cgi?id=13249



Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Reported-by: Werner Lemberg <wl@gnu.org>
Cc: Michal Januszewski <spock@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>