x86: replace a magic number with a named constant in the VESA boot code

Replace a magic number with a named constant in the VESA boot code.

Signed-off-by: Michal Januszewski <spock@gentoo.org>
Cc: linux-fbdev-devel@lists.sourceforge.net
Signed-off-by: Ingo Molnar <mingo@elte.hu>