drm/i915: Clear DRIVER_ATOMIC on a per-device basis

drm/i915: Clear DRIVER_ATOMIC on a per-device basis

Currently we're clearing DRIVER_ATOMIC in driver.driver_features
for older platforms. This will not work correctly should we ever
have a system with and old and new GPU in it. While that is not
possible currently let's make the code more correct and use
the per-device driver_features instead.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180913131622.17690-2-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>