drm/amd/display: fix regamma programming

When new coefficients match cached we would skip setting regamma mode
Also, when doing a stream update we would program regamma for all pipes,
even thos that are not yet powered on. This resulted in never setting
regamma since we would cache before the pipe is powered.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@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>