This adds zaphod and ZaphodHeads support
to the the in-server modesetting driver.

this is based on a request from Mario,
and on the current radeon driver, along
with some patches from Mario to bring things
up to the state of the art in Zaphod.

v2: fixup vblank fd registring.
v3: squash Mario's fixes.
  modesetting: Allow/Fix use of multiple ZaphodHead outputs per x-screen.
  modesetting: Take shift in crtc positions for ZaphodHeads configs into account.
  modesetting: Add ZaphodHeads description to man page.
small cleanups (airlied).

Reviewed-and-tested-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>