drm/amd/display: Revert "Set the power_down_on_boot function pointer to null"

drm/amd/display: Revert "Set the power_down_on_boot function pointer to null"

This reverts commit 54d9490314af ("drm/amd/display: Set the power_down_on_boot
function pointer to null")

[why & how]
This commit breaks S0i3 entry because DCN does not enter IPS2.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>