drm/amd/display: Change log level for DCN powergatting

Change log level from LOG_DC to LOG_DEBUG
LOG_DC spams dmesg on linux when hotpluging

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>