For compiling with allmodconfig, need vga.h file, so generate it which
just only include the asm-generic one.

It is firstly used by drivers/gpu/drm/drm_irq.c.

The related error:
include/video/vga.h:22:21: fatal error: asm/vga.h: No such file or directory
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>