drm/radeon/hdmi: use separated file for DCE 3.1/3.2 code

DCE 3.1 and 3.2 should be programmed in a different way than DCE 2 and
DCE 3. The order of setting registers and sets of registers are
different.
It's still unsure how we will handle DCE 3.1 vs. DCE 3.2, since they
have few differences as well.
For now separate DCE 2 and DCE 3 path, so we can work on it without a
risk of breaking DCE 3.1+.

This has been tested for possible regressions on DCE32 HD4550 (RV710).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>