backports: use old shrinkers API on old kernels

backports: use old shrinkers API on old kernels

The shrinkers API was changed in new kernel versions. This patch makes
the drm drivers use the old version on older kernel versions.

This backports the following commit from mainline kernel:
commit 1c1df1f6646854cca15fede54ec475b0e9f6a162
Author: Dave Chinner <dchinner@redhat.com>
Date:   Thu Jun 6 10:40:41 2013 +1000

    drivers: convert shrinkers to new count/scan API

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>