drm/radeon/evergreen/btc/fusion: setup hdp to invalidate and flush when asked

This needs to be explicitly set on btc.  It's set by default
on evergreen/fusion, so it fine to just unconditionally enable it for
all chips.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@gmail.com>