It seems that the content on 0x0c0000 is not the same as the vgabios Only the first (between Offset 0x00 and Offset 0x7f) words are the same. The rest differs from vgabios (Offset 0x80 - 0xFFFF). That is the reason why X detects a valid vga-Bios section on Offset 00 and 01 (55aa) and the rest fails.
Not all differs from the orginal vbios, but there are many parts in the image which are different. Also, the whole part, which should be FF.
chris