vga: fix incorrect line height in 640x200x2 mode

When in CGA modes, QEMU wants to ignore the maximum scan field (bits 0..4) of
the maximum scan length register in the CRTC.  It is not clear why this is
needed---for example, Bochs ignores bit 7 instead.  The issue is that the
CGA modes are not detected correctly, and in particular mode 6 results in
multi_scan==3 according to how SeaBIOS programs it.  The right way to check
for CGA graphics modes is to check whether bit 13 of the address is special
cased by the CRT controller to achieve line interleaving, i.e. whether bit 0
of the CRTC mode control register is clear.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1020


Reported-by: Korneliusz Osmenda <korneliuszo@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>