This just adds exa interfaces for mixed exa so drivers can
share and set shared pixmaps up correctly.
v2: update for passing slave screen.
v3: update for void *
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>