drm/amd/display: Enable power gating before init_pipes

[Why]
In init_hw() we call init_pipes() before enabling power gating.
init_pipes() tries to power gate dsc but it may fail because
required force-ons are not released yet.
As a result with dsc config the following errors observed on resume:
"REG_WAIT timeout 1us * 1000 tries - dcn20_dsc_pg_control"
"REG_WAIT timeout 1us * 1000 tries - dcn20_dpp_pg_control"
"REG_WAIT timeout 1us * 1000 tries - dcn20_hubp_pg_control"

[How]
Move enable_power_gating_plane() before init_pipes() in init_hw()

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Eric Yang <Eric.Yang2@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>