drm/amdgpu: don't evict vram on APUs for suspend to ram (v4)

Vram is system memory, so no need to evict.

v2: use PM_EVENT messages
v3: use correct dev
v4: use driver flags

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>