drm: bridge: dw-hdmi: Switch to regmap for register access

The Synopsys Designware HDMI TX Controller does not enforce register
access on platforms instanciating it. The current driver supports two
different types of memory-mapped flat register access, but in order to
support the Amlogic Meson SoCs integration, and provide a more generic
way to handle all sorts of register mapping, switch the register access
to use the regmap infrastructure.

In the case of registers that are not flat memory-mapped or do not
conform to the current driver implementation, a regmap struct can be
given in the plat_data and be used at probe or bind.

Since the AHB audio driver is only available with direct memory access,
only allow the I2S audio driver to be registered is directly
memory-mapped.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303172007.26541-10-laurent.pinchart+renesas@ideasonboard.com