drm/amd/display: Get DMUB registers from ASIC specific structs

[Why]
These values can differ per ASIC and should follow the full DC style
register programming model.

[How]
Define a common list and fill in the common list separately for
dcn20 and dcn21.

Unlike DC we're not using designated initializers for better compiler
compatibility since this resides in the DMUB service.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>