drm/amd/display: make dcn32_mpc_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_mpc.c:985:24: warning:
symbol 'dcn32_mpc_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>