drm/amd/display: make some functions static

drm/amd/display: make some functions static

These functions are not used outside the file dcn32_dccg.c, so the
modification is defined as static.

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_dccg.c:257:6: warning: no previous prototype for ‘dccg32_otg_drop_pixel’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_dccg.c:248:6: warning: no previous prototype for ‘dccg32_otg_add_pixel’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_dccg.c:211:6: warning: no previous prototype for ‘dccg32_set_dpstreamclk’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_dccg.c:136:6: warning: no previous prototype for ‘dccg32_set_dtbclk_dto’.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2142
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>