drm/lima: include used header file explicitly

To prevent build fail on some platform which does
not have it in the include file chain.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: a1d2a633 ("drm/lima: driver for ARM Mali4xx GPUs")
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190409003617.4470-2-yuq825@gmail.com

drivers/gpu/drm/lima/lima_gem.c

 // SPDX-License-Identifier: GPL-2.0 OR MIT
 /* Copyright 2017-2019 Qiang Yu <yuq825@gmail.com> */
 
+#include <linux/mm.h>
 #include <linux/sync_file.h>
 #include <linux/pfn_t.h>