drm/arc: Implement nonblocking commit correctly

Committing with block it is not.

Thanks to the fixed up vblank event handling we can just use the
helper support for nonblocking commits now.

Cc: Carlos Palminha <palminha@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: linux-snps-arc@lists.infradead.org
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-11-git-send-email-daniel.vetter@ffwll.ch