drm/amdgpu: Move doorbell structures to seperate file

Move doorbell structures, enum definitions and helper functions
from amdgpu.h to amdgpu_doorbell.h. No functional change

Signed-off-by: Oak Zeng <ozeng@amd.com>
Proposed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>