dim: restore cd in update_rerere_cache()

The refactoring dropped a cd to the drm-rerere dir, leading to "cp:
cannot stat 'rr-cache/*': No such file or directory" on dim ub.

Reported-by: Archit Taneja <architt@codeaurora.org>
Fixes: 599eb3db ("dim: Move all rerere updating into helpers")
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>