compositor-drm: drm_property_info_free() must reset

compositor-drm: drm_property_info_free() must reset

This function needs to reset the structures to NULL, otherwise it is not
possible to re-use a once "freed" property info array.

Being able to re-use an array is useful when the memory allocation and
array lifetimes do not match. A specific example is drm_output that is
changed to allocate the CRTC on enable() and deallocate it on disable().
A drm_output might be enabled and disabled multiple times.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>