Do not need to maintain our own shmem memory management
code as drm_gem_shmem_helpers provides it. And we can
also benifit from the work of others with shared code.

This is also a preparation for implementing buffer madv.
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010140152.17747-2-yuq825@gmail.com