drm/vmgfx: operation on ‘par->dirty.y1’ may be undefined

Trivial fix to set y1 = y2 = 0.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>