drm/qxl: add support for > 1 output

drm/qxl: add support for > 1 output

This adds support for a default of 4 heads, with a command line
parameter to change the default number.

It also overhauls the modesetting code to handle this case properly,
and send the correct things to the hardware at the right time.

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