drm/amdgpu: make display pinning more flexible (v2)

Only apply the static threshold for Stoney and Carrizo.
This hardware has certain requirements that don't allow
mixing of GTT and VRAM.  Newer asics do not have these
requirements so we should be able to be more flexible
with where buffers end up.

Bug: drm/amd#2270
Bug: drm/amd#2291
Bug: drm/amd#2255


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