drm/amdgpu: enable freesync for A+A configs

Some newer APUs can scanout directly from GTT, that saves us from
allocating another bounce buffer in VRAM and enables freesync in such
configurations.

Without this patch creating a framebuffer from the imported BO will
fail and userspace will fall back to a copy.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>