drm/amd/display: Add NULL check for local sink in edp_power_control

[WHY]
PNP cause bsod regression fix

[HOW]
Add NULL check

Signed-off-by: Yue Hin Lau <Yuehin.Lau@amd.com>
Reviewed-by: Hugo Hu <Hugo.Hu@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>