drm/vram-helper: Fix use of top-down placement

Commit 7053e0ea ("drm/vram-helper: stop using TTM placement flags")
cleared the BO placement flags if top-down placement had been selected.
Hence, BOs that were supposed to go into VRAM are now placed in a default
location in system memory.

Trying to scanout the incorrectly pinned BO results in displayed garbage
and an error message.

[  146.108127] ------------[ cut here ]------------
[  146.1V08180] WARNING: CPU: 0 PID: 152 at drivers/gpu/drm/drm_gem_vram_helper.c:284 drm_gem_vram_offset+0x59/0x60 [drm_vram_helper]
...
[  146.108591]  ast_cursor_page_flip+0x3e/0x150 [ast]
[  146.108622]  ast_cursor_plane_helper_atomic_update+0x8a/0xc0 [ast]
[  146.108654]  drm_atomic_helper_commit_planes+0x197/0x4c0
[  146.108699]  drm_atomic_helper_commit_tail_rpm+0x59/0xa0
[  146.108718]  commit_tail+0x103/0x1c0
...
[  146.109302] ---[ end trace d901a1ba1d949036 ]---

Fix the bug by keeping the placement flags. The top-down placement flag
is stored in a separate variable.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Fixes: 7053e0ea ("drm/vram-helper: stop using TTM placement flags")
Cc: Christian König <christian.koenig@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
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/20200921142536.4392-1-tzimmermann@suse.de

drivers/gpu/drm/drm_gem_vram_helper.c

@@ -140,7 +140,7 @@ static void drm_gem_vram_placement(struct drm_gem_vram_object *gbo,
 	unsigned int c = 0;
 
 	if (pl_flag & DRM_GEM_VRAM_PL_FLAG_TOPDOWN)
-		pl_flag = TTM_PL_FLAG_TOPDOWN;
+		invariant_flags = TTM_PL_FLAG_TOPDOWN;
 
 	gbo->placement.placement = gbo->placements;
 	gbo->placement.busy_placement = gbo->placements;