drm: factor out drm_close_helper() function

Will be used to plug an existing memory leak.

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