drm/amdgpu: add apu flags (v2)

Add some APU flags to simplify handling of different APU
variants.  It's easier to understand the special cases
if we use names flags rather than checking device ids and
silicon revisions.

v2: rebase on latest code

Acked-by: Evan Quan <evan.quan@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>