drm/radeon/dce6: fix display powergating

commit 0e3d50bfcbd338254795a700dcff429a96cba1a6 upstream.

Only enable it when we disable the display rather than
at DPMS time since enabling it requires a full modeset
to restore the display state.  Fixes blank screens in
certain cases.

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