drm/i915/display: Fix build error without DRM_FBDEV_EMULATION

drm/i915/display: Fix build error without DRM_FBDEV_EMULATION

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@huawei.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/r/20250315120143.2344958-1-yuehaibing@huawei.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>