drm/msm: Clear msm_obj->sgt in put_pages()

Currently this doesn't matter since we keep the pages pinned until the
object is destroyed.  But when we start unpinning pages to allow objects
to be evicted to swap, it will.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210405174532.1441497-4-robdclark@gmail.com
Signed-off-by: Rob Clark <robdclark@chromium.org>