drm: Implement O_NONBLOCK support on /dev/dri/cardN

The implmentation is simple in the extreme: we only want to wait for
events if the device was opened in blocking mode, otherwise we grab what
is available and report an error if there was none.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Testcase: igt/kms_flip/nonblocing_read
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>