[PATCH -next] drm/i915/display: Fix build error without DRM_FBDEV_EMULATION

[Yue Haibing <yuehaibing@xxxxxxxxxx>]

In file included from <command-line>:
./drivers/gpu/drm/i915/display/intel_fbdev.h: In function ‘intel_fbdev_framebuffer’:
./drivers/gpu/drm/i915/display/intel_fbdev.h:32:16: error: ‘NULL’ undeclared (first use in this function)
   32 |         return NULL;
      |                ^~~~
./drivers/gpu/drm/i915/display/intel_fbdev.h:1:1: note: ‘NULL’ is defined in header ‘<stddef.h>’; did you forget to ‘#include <stddef.h>’?
  +++ |+#include <stddef.h>
    1 | /* SPDX-License-Identifier: MIT */
./drivers/gpu/drm/i915/display/intel_fbdev.h:32:16: note: each undeclared identifier is reported only once for each function it appears in
   32 |         return NULL;
      |                ^~~~

Build fails if CONFIG_DRM_FBDEV_EMULATION is n, add missing header file.

Fixes: 9fa154f40eb6 ("drm/{i915,xe}: Run DRM default client setup")
Signed-off-by: Yue Haibing <yuehaibing@xxxxxxxxxx>
---
 drivers/gpu/drm/i915/display/intel_fbdev.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_fbdev.h b/drivers/gpu/drm/i915/display/intel_fbdev.h
index ca2c8c438f02..89bad3a2b01a 100644
--- a/drivers/gpu/drm/i915/display/intel_fbdev.h
+++ b/drivers/gpu/drm/i915/display/intel_fbdev.h
@@ -6,6 +6,8 @@
 #ifndef __INTEL_FBDEV_H__
 #define __INTEL_FBDEV_H__
 
+#include <linux/types.h>
+
 struct drm_fb_helper;
 struct drm_fb_helper_surface_size;
 struct drm_i915_private;
-- 
2.34.1