drm: bridge/dw_hdmi: clean up HDMI vs DVI mode handling

The FSL kernel detects the HDMI vendor id, and uses this to set
hdmi->edid_cfg.hdmi_cap, which is then used to set mdvi appropriately,
rather than detecting whether we are outputting a CEA mode.  Update
the dw_hdmi code to use this logic, but lets eliminate the mdvi
variable, prefering the more verbose "hdmi->sink_is_hdmi" instead.

Use the generic drm_detect_hdmi_monitor() to detect a HDMI sink.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>