drm/amd/display: make enc32_stream_encoder_dvi_set_stream_attribute static

This symbol is not used outside of dcn32_dio_stream_encoder.c, so marks
it static.

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_dio_stream_encoder.c:63:6: warning: no previous prototype for ‘enc32_stream_encoder_dvi_set_stream_attribute’.

Link:https://bugzilla.openanolis.cn/show_bug.cgi?id=2119


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>