Include the drm_plane_helper.h header file to fix the following sparse
warnings:

	  CHECK   drivers/gpu/drm/drm_plane_helper.c
	drivers/gpu/drm/drm_plane_helper.c:102:5: warning: symbol 'drm_primary_helper_update' was not declared. Should it be static?
	drivers/gpu/drm/drm_plane_helper.c:219:5: warning: symbol 'drm_primary_helper_disable' was not declared. Should it be static?
	drivers/gpu/drm/drm_plane_helper.c:233:6: warning: symbol 'drm_primary_helper_destroy' was not declared. Should it be static?
	drivers/gpu/drm/drm_plane_helper.c:241:30: warning: symbol 'drm_primary_helper_funcs' was not declared. Should it be static?
	drivers/gpu/drm/drm_plane_helper.c:259:18: warning: symbol 'drm_primary_helper_create_plane' was not declared. Should it be static?

Doing that makes gcc complain as follows:

	  CC      drivers/gpu/drm/drm_plane_helper.o
	drivers/gpu/drm/drm_plane_helper.c:260:19: error: conflicting types for 'drm_primary_helper_create_plane'
	 struct drm_plane *drm_primary_helper_create_plane(struct drm_device *dev,
	                   ^
	In file included from drivers/gpu/drm/drm_plane_helper.c:29:0:
	include/drm/drm_plane_helper.h:42:19: note: previous declaration of 'drm_primary_helper_create_plane' was here
	 struct drm_plane *drm_primary_helper_create_plane(struct drm_device *dev,
	                   ^
	drivers/gpu/drm/drm_plane_helper.c: In function 'drm_primary_helper_create_plane':
	drivers/gpu/drm/drm_plane_helper.c:274:11: warning: assignment discards 'const' qualifier from pointer target type
	   formats = safe_modeset_formats;
	           ^
	In file included from include/linux/linkage.h:6:0,
	                 from include/linux/kernel.h:6,
	                 from include/drm/drmP.h:45,
	                 from drivers/gpu/drm/drm_plane_helper.c:27:
	drivers/gpu/drm/drm_plane_helper.c: At top level:
	drivers/gpu/drm/drm_plane_helper.c:289:15: error: conflicting types for 'drm_primary_helper_create_plane'
	 EXPORT_SYMBOL(drm_primary_helper_create_plane);
	               ^
	include/linux/export.h:57:21: note: in definition of macro '__EXPORT_SYMBOL'
	  extern typeof(sym) sym;     \
	                     ^
	drivers/gpu/drm/drm_plane_helper.c:289:1: note: in expansion of macro 'EXPORT_SYMBOL'
	 EXPORT_SYMBOL(drm_primary_helper_create_plane);
	 ^
	In file included from drivers/gpu/drm/drm_plane_helper.c:29:0:
	include/drm/drm_plane_helper.h:42:19: note: previous declaration of 'drm_primary_helper_create_plane' was here
	 struct drm_plane *drm_primary_helper_create_plane(struct drm_device *dev,
	                   ^

Which can easily be fixed by making the signatures of the implementation
and the prototype match.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>