drm/amd/display: make dcn32_mmhubbub_funcs static

drm/amd/display: make dcn32_mmhubbub_funcs static

The symbol is not used outside of the file, so mark it static.

Fixes the following warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn32/dcn32_mmhubbub.c:214:28:
warning: symbol 'dcn32_mmhubbub_funcs' was
not declared. Should it be static?

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>