drm/amd/display: make some functions static

These functions are not used outside the file dcn32_hubbub.c, so the
modification is defined as static.

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn314/dcn314_dccg.c:223:6: warning: no previous prototype for ‘dccg314_set_valid_pixel_rate’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn314/dcn314_dccg.c:184:6: warning: no previous prototype for ‘dccg314_set_dpstreamclk’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn314/dcn314_dccg.c:140:6: warning: no previous prototype for ‘dccg314_set_dtbclk_dto’.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2144


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>