drm/amd/display: Make some functions static

Fix the following sparse warnings:

drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_resource.c:483:21: warning: symbol 'dce120_clock_source_create' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_resource.c:506:6: warning: symbol 'dce120_clock_source_destroy' was not declared. Should it be static?
drivers/gpu/drm/amd/amdgpu/../display/dc/dce120/dce120_resource.c:513:6: warning: symbol 'dce120_hw_sequencer_create' was not declared. Should it be static?

Fixes: b8fdfcc6 ("drm/amd/display: Add DCE12 core support")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai26@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>