drm: bridge: dw-hdmi: Create PHY operations

The HDMI TX controller support different PHYs whose programming
interface can vary significantly, especially with vendor PHYs that are
not provided by Synopsys. To support them, create a PHY operation
structure that can be provided by the platform glue layer. The existing
PHY handling code (limited to Synopsys PHY support) is refactored into a
set of default PHY operations that are used automatically when the
platform glue doesn't provide its own operations.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170305233615.11993-1-laurent.pinchart+renesas@ideasonboard.com