Simplify the driver code with DRM GEM helper function.

v2:
improve commit comment.
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010140152.17747-3-yuq825@gmail.com