drm/radeon/kms: restore surface registers on resume.

On resume on my rv530 laptop surface cntl was left disabled, so
wierd stuff would happen with rendering to a tiled front buffer.

This checks if the surface regs are assigned to bos and reprograms
the surface registers on resume using the same path that clears
them all on init.

Signed-off-by: Dave Airlie <airlied@redhat.com>