drm: Resurrect atomic rmfb code

This was somehow lost between v3 and the merged version in Maarten's
patch merged as:

commit f2d580b9
Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Wed May 4 14:38:26 2016 +0200

    drm/core: Do not preserve framebuffer on rmfb, v4.

Actual code copied from Maarten's patch, but with the slight change to
just use dev->mode_config.funcs->atomic_commit to decide whether to
use the atomic path or not.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-24-git-send-email-daniel.vetter@ffwll.ch