drm/amd/display: Fix time out on boot

On boot, hubp 0 is powergated during enable accel mode, so we time out
when we try to blank in undo wa.

Fix: Check power gate status before set blank

Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>