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

This reverts commit 54d94903 ("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>

drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_init.c

@@ -39,7 +39,7 @@
 static const struct hw_sequencer_funcs dcn35_funcs = {
 	.program_gamut_remap = dcn30_program_gamut_remap,
 	.init_hw = dcn35_init_hw,
-	.power_down_on_boot = NULL,
+	.power_down_on_boot = dcn35_power_down_on_boot,
 	.apply_ctx_to_hw = dce110_apply_ctx_to_hw,
 	.apply_ctx_for_surface = NULL,
 	.program_front_end_for_ctx = dcn20_program_front_end_for_ctx,