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>