drm/amd/display: Separate mem input constuctors for dce 112 and 120

Override the memory input functions for dce120 not to program
new registers on dce112.
This will fix warnings thrown on Polaris asics.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>