drm/hdlcd: Use helper support for nonblocking commits

With the fixed up drm event handling for crtc_state->event we can just
use the helper support for nonblocking commits.

Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Tested-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465388359-8070-12-git-send-email-daniel.vetter@ffwll.ch