drm/shmem: add support for per object caching flags.

Add map_cached bool to drm_gem_shmem_object, to request cached mappings
on a per-object base.  Check the flag before adding writecombine to
pgprot bits.

Cc: stable@vger.kernel.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Guillaume Gardet <Guillaume.Gardet@arm.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200226154752.24328-2-kraxel@redhat.com

include/drm/drm_gem_shmem_helper.h

@@ -96,6 +96,11 @@ struct drm_gem_shmem_object {
 	 * The address are un-mapped when the count reaches zero.
 	 */
 	unsigned int vmap_use_count;
+
+	/**
+	 * @map_cached: map object cached (instead of using writecombine).
+	 */
+	bool map_cached;
 };
 
 #define to_drm_gem_shmem_obj(obj) \