drm/amd/display: make dcn32_link_encoder_is_in_alt_mode and dcn32_link_encoder_get_ma...

drm/amd/display: make dcn32_link_encoder_is_in_alt_mode and dcn32_link_encoder_get_max_link_cap static

These two functions are not used outside the function
dcn32_dio_link_encoder.c, so the modification is defined as static.

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_dio_link_encoder.c:121:6: warning: no previous prototype for ‘dcn32_link_encoder_is_in_alt_mode’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_dio_link_encoder.c:136:6: warning: no previous prototype for ‘dcn32_link_encoder_get_max_link_cap’.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2117
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>