drm/amd/display: display connected to dp-1 does not light up

[why]
for vega, dp set_panel_mode is
handled by psp firmware. dal should not program the
register again.

[how]
dal does not program panel mode.

Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>