drm/amdgpu: header cleanup

No function change, just move a bunch of definitions from amdgpu.h into
separate header files.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>