drm/i915: Add the exclusive fence to plane_state.

Set plane_state->base.fence to the dma_buf exclusive fence,
and add a wait to the mmio function. This will make it easier
to unify plane updates later on.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463490484-19540-11-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>