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/dcn32/dcn32_hubbub.c:912:6: warning: no previous prototype for ‘hubbub32_force_wm_propagate_to_pipes’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_hubbub.c:823:6: warning: no previous prototype for ‘hubbub32_wm_read_state’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_hubbub.c:772:6: warning: no previous prototype for ‘hubbub32_init_watermarks’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_hubbub.c:712:6: warning: no previous prototype for ‘hubbub32_force_usr_retraining_allow’.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2141


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>