drm/radeon/kms: fix LVDS setup on r4xx

R4xx mobility chips use atombios, which does not store
the LVDS_GEN_CNTL parameter setup like combios.  Rather,
it's configured in LVDSEncoderControl.  As such,
LVDS_GEN_CNTL is set wrong when on resume.  Call
LVDSEncoderControl to set it properly.

Should fix fdo bug 25336

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