drm/amd/display: change to enc314_stream_encoder_dp_blank static

enc314_stream_encoder_dp_blank is only used in dcn314_dio_stream_encoder.c now,
change it to static.

Fixes: c55bf690

 ("drm/amd/display: Add explicit FIFO disable for DP blank")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>