drm: dw-hdmi: Fix compilation breakage by selecting REGMAP_MMIO

The dw-hdmi driver switched to regmap-mmio, but lacks the dependency in
Kconfig. This can result in compilation breakages. Fix it by selecting
REGMAP_MMIO.

Fixes: 80e2f979 ("drm: bridge: dw-hdmi: Switch to regmap for register access")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170610085943.15788-1-laurent.pinchart+renesas@ideasonboard.com

drivers/gpu/drm/bridge/synopsys/Kconfig

 config DRM_DW_HDMI
 	tristate
 	select DRM_KMS_HELPER
+	select REGMAP_MMIO
 
 config DRM_DW_HDMI_AHB_AUDIO
 	tristate "Synopsys Designware AHB Audio interface"