DRM/Radeon: On DVI-I use Load Detection when EDID is bogus.

The Radeon driver uses the analog/digital flag to determine if the
DAC or the TMDS encoder should be enabled on a DVI-I connector.
If the EDID is bogus this flag is no longer reliable. This fix
adds a fallback to DAC load detection to determine if anything
is connected to the DAC. If not and a (bogus) EDID is found it
assumes a digital display is connected.
This works around problems with some crappy IPMI devices using
Radeon ES1000.

Signed-off-by: Egbert Eich <eich@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>