drm/radeon: fix HDP flushing

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>
Cc: stable@vger.kernel.org
Signed-off-by: Grigori Goronzy <greg@chown.ath.cx>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/radeon_gem.c
Simple merge