drm/I915: Use the CRT DDC to get the EDID for DVI-connector on Mac

mac Mini's have a single DDC line on the DVI connector, shared between the
analog link and the digital link. So, if DDC isn't detected on GPIOE (the
usual SDVO DDC link), try GPIOA (the usual VGA DDC link) when there isn't a
VGA monitor connected.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>