drm/radeon: fix HDP flushing

drm/radeon: fix HDP flushing

commit 54e03986133468e02cb01b76215e4d53a9cf6380 upstream.

This was regressed by commit 39e7f6f8, although I don't know of any
actual issues caused by it.

The storage domain is read without TTM locking now, but the lock
never helped to prevent any races.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Grigori Goronzy <greg@chown.ath.cx>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>