drm/hisilicon: Add support for external bridge

Add support for external HDMI bridge.

v8: None.
v7: None.
v6: None.
v5: None.
v4: None.
v3:
- Fix a typo: s/exteranl/external.
v2:
- Remove abtraction layer.

Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Signed-off-by: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>