vbe: Add VBE 2.0+ OemData field to struct vbe_info

vbe: Add VBE 2.0+ OemData field to struct vbe_info

Per the VBE 2.0 specification, the VBE controller information is 512
bytes long when the "VBE2" signature is provided, instead of the
original 256 bytes.

src/bootsplash.c uses the original pre-VBE-2.0 256-byte structure while
also filling in the "VBE2" signature, so a video BIOS that makes use of
the VBE2 OemData area could write past the end of the allocated region.

The original bootsplash code did not have this bug; it was introduced
when the bootsplash VBE structures were merged with the VGA ROM struct
definitions.

Fixes: 69e941c159ed ("Merge bootsplash and VGA ROM vbe structure definitions")
Signed-off-by: Daniel Verkamp <daniel@drv.nu>