drm/radeon/kms: fix connector edid handling

Based partly on a patch from
Christian Koenig <deathsimple@vodafone.de>

- fix several memory leaks in radeon_connector->edid handling
- store edid in radeon_connector->edid in detect() or get_modes()
- switch hdmi detect code to use radeon_connector->edid
- add support for oem boards multiple connectors that share
a ddc line.
- short circuit lvds_detect() if have a stored edid

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>