drm/amd/display: Make dcn35_hubp_funcs static

The sparse tool complains as follows:

drivers/gpu/drm/amd/amdgpu/../display/dc/hubp/dcn35/dcn35_hubp.c:191:19: warning:
	symbol 'dcn35_hubp_funcs' was not declared. Should it be static?

This symbol is not used outside of dcn35_hubp.c, so marks it static.

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