drm/amd/amdgpu: update GC 10.3.0 pwrdec

The 10.3 GC headers were missing most of the pwrdec block.
This patch adds the registers and bits present in the 10.1 header
but based on the contents of the 10.3 specs.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>