drm: refactor getproperties/getconnector

Both need to iterate a mode objects properties.  Split that out into a
helper shared by both ioctl handlers, since this is going to become more
complicated when we add atomic properties (which will need filtering
from legacy userspace).

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>