drm/amd/display: fix dcn pipe reset sequence

This change fixes dcn10 front end reset sequence. Previously we
would reset front end during flip which led to issues
in certain MPO and 4k/5k scenarios. We would also never properly
power gate our front end.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>