drm/amd/display: make optc314_phantom_crtc_post_enable static

drm/amd/display: make optc314_phantom_crtc_post_enable static

This symbol is not used outside of dcn314_optc.c, so marks it static.

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn314/dcn314_optc.c:153:6: warning: no previous prototype for ‘optc314_phantom_crtc_post_enable’.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2147
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>