drm/amd/display: Guard reading 3DLUT registers for dcn32/dcn35

drm/amd/display: Guard reading 3DLUT registers for dcn32/dcn35

[Why]
3DLUT is not part of the DPP on DCN32/DCN35 ASIC and these registers
now exist in MCM state.

[How]
Add guards when reading DPP state based on whether the register has a
valid offset.

Reviewed-by: Sung joon Kim <sungjoon.kim@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>