drm/amdgpu/gfx8: set doorbell range for polaris as well

Add missing chips to the doorbell range setup.  These
were missed in the KIQ code.  Fixes power and performance
regressions with KIQ.  Spotted by Rex.

Tested-and-Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>