UPSTREAM: 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


(cherry picked from commit e21710a893c8705360b764fa44061d72d65fb2ae)
Bug: 152417756
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: Id2ec9a880b3a636e2d1d73d85cb5ff2e8614725e