drm/amd/display: Remove some unused definitions from DCN32/321

[Why&How]
After reg offset initialization was switched to runtime rather than
compile time, some of the defintions are not needed anymore and can
be removed.

Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>