drm/amdgpu/virt: fix handling of the atomic flag

Use the per device drm driver feature flags rather than the
global one.  This way we can make the drm driver struct const.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201104100425.1922351-3-daniel.vetter@ffwll.ch