drm/radeon/hdmi: DCE2: update setmode

Recent RE efforts revealed ops performed by fglrx during HDMI setup.
This mostly adds masks to r/w ops plus few single missing bits.

This has been tested for possible regressions on:
1) DCE2 HD2400 (RV610)
2) DCE3 HD3470 (RV620)

For a reference and details see:
https://bugzilla.kernel.org/show_bug.cgi?id=76231



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