drm/amd/display: make some functions static

drm/amd/display: make some functions static

These functions are not used outside the file dcn30_resource.c, so the
modification is defined as static.

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn30/dcn30_resource.c:1949:6: warning: no previous prototype for ‘is_refresh_rate_support_mclk_switch_using_fw_based_vblank_stretch’.
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn30/dcn30_resource.c:1919:5: warning: no previous prototype for ‘get_frame_rate_at_max_stretch_100hz’.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2143
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>