drm/amd/display: Change frontend/backend programming sequence

This is a follow-up to the following change:

Yongqiang Sun: Program front end first when set mode.

Due to pipe-splitting features, how we handle stream enabling and
disabling needs to change.

In the case of pipe split disable, two planes need to be combined back
into the same stream. This needs to be done before any stream
programming happens.

The previous patch addresses this, but breaks cross-platform
compatibility. It's not guaranteed that a dc commit will be called
separately to program planes and streams.

Therefore, we handle the combined commit case by doing plane programming
both before and after stream programming, to handle pipe split disable
and plane enable respectively.

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>