drm/amdgpu: handle polaris10/11 overlap asics (v2)

Some special polaris 10 chips overlap with the polaris11
DID range.  Handle this properly in the driver.

v2: use local flags for other function calls.

Acked-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org