drm: factor out drm_close_helper() function

Will be used to plug an existing memory leak.

Change-Id: I7f482c6ae3db07301dee27158d9b6d7a5aa5a93e
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114084410.15266-1-emil.l.velikov@gmail.com
Git-commit: e21710a893c8705360b764fa44061d72d65fb2ae
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>