drm/amd/display: default force_single_disp_pipe_split = 1 on RV

1080p idle, stutter efficiency goes up from 95.8% to 97.8%
result in 5mW saving from APU and 8mW saving from DDR4

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>