drm/radeon/kms: fix regression in RMX code (v2)

caused by d65d65b1

need to update the radeon crtc priv native mode before using it.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=30049



v2: integrate v/h copy paste typo

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