dim: move $time as global $dim_timestamp

time was local in dim_rebuild_tip, so not too nice to reference in
functions called from it.

Reported-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>