drm/radeon/kms: re-emit full context state for evergreen blits

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>