drm/amdgpu: add amdgpu_reg_state.h

This header defines the reg state structures exposed via
sysfs for umr debugging.

v2: add content type

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>