drm/edid: Retry EDID fetch up to four times

This matches the X server's retry logic.  Note that we'll only retry if
we get a DDC response but fail validation; legitimately disconnected
outputs will bomb out early.

See also: http://bugzilla.redhat.com/532957



Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>