drm/radeon/kms: move panel mode setup into encoder mode set

commit 386d4d75 upstream.

Needs to happen earlier in the mode set.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>