drm/amd/display: Check power_down functions exist before calling

[WHY]
The power_down() function was only defined for specific asics and will
crash the system if it is called by an asic with eDP connected that does
not have it defined.

[HOW]
Add a check for the function's existence before calling it.
Signed-off-by: Sung Lee <sung.lee@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c

@@ -1379,7 +1379,10 @@ void dcn10_init_hw(struct dc *dc)
 
 		if (edp_link &&
 				edp_link->link_enc->funcs->is_dig_enabled &&
-			edp_link->link_enc->funcs->is_dig_enabled(edp_link->link_enc)) {
+				edp_link->link_enc->funcs->is_dig_enabled(edp_link->link_enc) &&
+				dc->hwss.edp_backlight_control &&
+				dc->hwss.power_down &&
+				dc->hwss.edp_power_control) {
 			dc->hwss.edp_backlight_control(edp_link, false);
 			dc->hwss.power_down(dc);
 			dc->hwss.edp_power_control(edp_link, false);