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>

drivers/gpu/drm/amd/display/dc/dcn32/dcn32_mpc.c

@@ -982,7 +982,7 @@ static bool mpc32_program_3dlut(
 	return true;
 }
 
-const struct mpc_funcs dcn32_mpc_funcs = {
+static const struct mpc_funcs dcn32_mpc_funcs = {
 	.read_mpcc_state = mpc1_read_mpcc_state,
 	.insert_plane = mpc1_insert_plane,
 	.remove_mpcc = mpc1_remove_mpcc,