drm/amdgpu/display: clean up dcn2*_pp_smu functions

Use the dcn21 functions in dcn21_resource.c and make the
dcn20 functions static since they are only used in
dcn20_resource now.

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