drm/radeon/kms/evergreen: reset the grbm blocks at resume and init

This fixes module reloading and resume as the gfx block seems to
be left in a bad state in some cases.

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