drm/amd/display: Ungate stream before programming registers

[Why]
Certain tests fail after a fresh reboot. This is caused by writing to
registers prior to ungating the stream we're trying to program.

[How]
Make sure the stream is ungated before writing to its registers.
This also enables power-gating plane resources before init_hw
initializes them.
Additionally, this does some refactoring to move gating/ungating
from enable/disable_plane functions to where stream resources are
enabled/disabled.

Signed-off-by: Gary Kattan <gary.kattan@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>