drm/i915: Separate intel_frontbuffer into its own header

In view of adding inline functions into the intel_frontbuffer section,
we first split the header into its own file so that we can integrate it
more easily with kerneldoc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-19-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/intel_sprite.c b/drivers/gpu/drm/i915/intel_sprite.c
index 0de935a..e045295 100644
--- a/drivers/gpu/drm/i915/intel_sprite.c
+++ b/drivers/gpu/drm/i915/intel_sprite.c
@@ -36,6 +36,7 @@
 #include <drm/drm_atomic.h>
 #include <drm/drm_plane_helper.h>
 #include "intel_drv.h"
+#include "intel_frontbuffer.h"
 #include <drm/i915_drm.h>
 #include "i915_drv.h"