drm/amd/display: make get_color_space_type() static

This addresses the following sparse warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc_hw_sequencer.c:180:26:
warning: symbol 'get_color_space_type' was not declared. Should it be
static?

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>