drm/amd/display: make optc32_phantom_crtc_post_enable, optc32_setup_manual_trigger and optc32_set_drr static

These three functions are not used outside the function
dcn32_optc.c, so the modification is defined as static.

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_optc.c:159:6: warning: no previous prototype for function 'optc32_phantom_crtc_post_enable'.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_optc.c:218:6: warning: no previous prototype for ‘optc32_set_drr’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_optc.c:193:6: warning: no previous prototype for ‘optc32_setup_manual_trigger’.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2140Reported-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>