hw/display: Add basic ATI VGA emulation

hw/display: Add basic ATI VGA emulation

At least two machines, the PPC mac99 and MIPS fulong2e, have an ATI
gfx chip by default (Rage 128 Pro and M6/RV100 respectively) and
guests running on these and the PMON2000 firmware of the fulong2e
expect this to be available. Fortunately these are very similar chips
so they can be mostly emulated in the same device model. This patch
adds basic emulation of these ATI VGA chips.

While this is incomplete and currently only enough to run the MIPS
firmware and get framebuffer output with Linux, it allows the fulong2e
board to work more like the real hardware and having it in QEMU in
this state provides a way to experiment with it and allows others to
contribute to improve it. It is compiled for all archs but only the
fulong2e (which currently has no display output at all) is set to use
it by default (in a separate patch).

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Tested-by: Andrew Randrianasulu <randrianasulu@gmail.com>
Tested-by: Howard Spoelstra <hsp.cat7@gmail.com>
Message-id: 0b1b7c22873a6e37627261b04fb687412b25ff4f.1552152100.git.balaton@eik.bme.hu
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>