drm/i915/buddy: adjust res->start

Differentiate between mappable vs non-mappable resources, also if this
is an actual range allocation ensure we set res->start as the starting
pfn. Later when we need to do non-mappable -> mappable moves then we
want TTM to see that the current placement is not compatible, which
should result in an actual move, instead of being turned into a noop.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220225145502.331818-5-matthew.auld@intel.com

drivers/gpu/drm/i915/i915_ttm_buddy_manager.c

@@ -141,6 +141,13 @@ static int i915_ttm_buddy_man_alloc(struct ttm_resource_manager *man,
 		mutex_unlock(&bman->lock);
 	}
 
+	if (place->lpfn - place->fpfn == n_pages)
+		bman_res->base.start = place->fpfn;
+	else if (lpfn <= bman->visible_size)
+		bman_res->base.start = 0;
+	else
+		bman_res->base.start = bman->visible_size;
+
 	*res = &bman_res->base;
 	return 0;