drm/ttm: Fix an invalid freeing on already freed page in error path

If caching mode change fails due to, for example, OOM we
free the allocated pages in a two-step process. First the pages
for which the caching change has already succeeded. Secondly
the pages for which a caching change did not succeed.

However the second step was incorrectly freeing the pages already
freed in the first step.

Fix.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Fixes: 379989e ("drm/ttm/pool: Fix ttm_pool_alloc error path")
Cc: Christian König <christian.koenig@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.4+
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240221073324.3303-1-thomas.hellstrom@linux.intel.com

Author: Thomas Hellström

drivers/gpu/drm/ttm/ttm_pool.c

@@ -387,7 +387,7 @@ static void ttm_pool_free_range(struct ttm_pool *pool, struct ttm_tt *tt,
 				enum ttm_caching caching,
 				pgoff_t start_page, pgoff_t end_page)
 {
-	struct page **pages = tt->pages;
+	struct page **pages = &tt->pages[start_page];
 	unsigned int order;
 	pgoff_t i, nr;