drm: Replace fb_helper->atomic with mode_config->atomic_commit

Drivers transitioning to atomic might not yet want to enable full
DRIVER_ATOMIC support when it's not entirely working. But using atomic
internally makes a lot more sense earlier.

Instead of spreading such flags to more places I figured it's simpler
to just check for mode_config->funcs->atomic_commit, and use atomic
paths if that is set. For the only driver currently transitioning
(i915) this does the right thing.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-23-git-send-email-daniel.vetter@ffwll.ch