drm/amd/display: Constify dcn21_res_pool_funcs

The only usage of dcn21_res_pool_funcs is to assign its address to a
const pointer. Make it const to allow the compiler to put it in
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>