drm/amd/display: Make dcn401_dsc_funcs static

The sparse tool complains as follows:

drivers/gpu/drm/amd/amdgpu/../display/dc/dsc/dcn401/dcn401_dsc.c:30:24: warning:
	symbol 'dcn401_dsc_funcs' was not declared. Should it be static?

This symbol is not used outside of dcn401_dsc.c, so marks it static.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>