drm/amdgpu: Add initial VI support

This adds initial support for VI asics.  This
includes Iceland, Tonga, and Carrizo.  Our inital
focus as been Carrizo, so there are still gaps in
support for Tonga and Iceland, notably power
management.

Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Jammy Zhou <Jammy.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>