drm/gud: Fix missing include

Add missing vmalloc.h include.

Fixes: c17d0486 ("drm/gud: Use the shadow plane helper")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221207-gud-missing-include-v1-0-c5b32c9c59da@tronnes.org