drm: bridge/dw_hdmi: add atomic API support

Fill atomic needed funcs with default atomic helper library.

Rockchip use dw_hdmi, and drm/rockchip will covert to atomic api,
we need dw_hdmi support atomic funcs.

Now another drm driver use dw_hdmi is imx, not yet atomic, so
check DRIVER_ATOMIC at runtime to spilt atomic and not atomic.

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>