clear state doesn't seem to work properly in some cases

Fixes hangs in heavy 3D on some evergreen cards reported on
IRC.

May fix:
https://bugs.freedesktop.org/show_bug.cgi?id=33381
possibly others.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@gmail.com>