mgag200: initial g200se driver (v2)

This is a driver for the G200 server engines chips,
it doesn't driver any of the Matrix G series desktop cards.

It will bind to G200 SE A,B, G200EV, G200WB, G200EH and G200ER cards.

Its based on previous work done my Matthew Garrett but remodelled
to follow the same style and flow as the AST server driver. It also
works along the same lines as the AST server driver wrt memory management.

There is no userspace driver planned, xf86-video-modesetting should be used.
It also appears these GPUs have no ARGB hw cursors.

v2: add missing tagfifo reset + G200 SE memory bw setup pieces.

Signed-off-by: Dave Airlie <airlied@redhat.com>