drm: Include <linux/mem_encrypt.h> in drm_cache.c

The function drm_need_swiotbl() needs mem_encrypt_active() from
<linux/mem_encrypt.h>. The include got lost when refactoring the
code recently.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 3abc6670

 ("drm: Implement drm_need_swiotlb() in drm_cache.c")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Christian König <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210114080535.17132-1-tzimmermann@suse.de