drm/amdgpu: Use per-device driver_features to disable atomic

drm/amdgpu: Use per-device driver_features to disable atomic

Disable atomic on a per-device basis instead of for all devices.
Made possible by the new device.driver_features thing.

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Michel Dänzer <michel@daenzer.net>
Suggested-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180913163147.27900-2-ville.syrjala@linux.intel.com
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>