drm/i915/ttm: use cached system pages when evicting lmem

This should let us do an accelerated copy directly to the shmem pages
when temporarily moving lmem-only objects, where the i915-gem shrinker
can later kick in to swap out the pages, if needed.
Signed-off-by: NMatthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: NThomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211018091055.1998191-7-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/i915_gem_ttm.c

@@ -134,11 +134,11 @@ static enum ttm_caching
 i915_ttm_select_tt_caching(const struct drm_i915_gem_object *obj)
 {
 	/*
-	 * Objects only allowed in system get cached cpu-mappings.
-	 * Other objects get WC mapping for now. Even if in system.
+	 * Objects only allowed in system get cached cpu-mappings, or when
+	 * evicting lmem-only buffers to system for swapping. Other objects get
+	 * WC mapping for now. Even if in system.
 	 */
-	if (obj->mm.region->type == INTEL_MEMORY_SYSTEM &&
-	    obj->mm.n_placements <= 1)
+	if (obj->mm.n_placements <= 1)
 		return ttm_cached;
 
 	return ttm_write_combined;