drm/radeon/kms: Use the standard VESA timeout for DDC channels

The VESA specification suggests a 2.2 ms timeout on DDC channels.
Use exactly that (as the i915 driver does) instead of hard-coding a
jiffy count.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>