drm/radeon/kms/evergreen: set the clear state to the blit state

The hw stores a default clear state for registers in the context
range that can be initialized when the CP is set up.  Set the
blit state as the default clear state and use the CLEAR_STATE
packet to load the blit state rather than loading it from an IB.
This reduces overhead when doing bo moves using the 3D engine.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>