drm/amd/amdgpu: Include headers for PWR and SMUIO registers

Clean up the smu10, smu12, and gfx9 drivers to use headers for
registers instead of hardcoding in the C source files.

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>