Partially revert "drm/i915: Use full atomic modeset."

This partially reverts commit 74c090b1.

The DRIVER_ATOMIC cap cannot yet be exported because i915 lacks async
support.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>