drm/amdgpu: add some required DCE6 registers (v7)

To help with the DC port.

v2: add missing masks, add additional registers
v3: more updates
v4: fix accidently dropped changes
v5: add missing nb pstate mask
v6: add vblank, vline masks
v7: add SCL_HORZ_FILTER_INIT regs

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>