drm: Kill drm perf counter leftovers

The user of these counters was killed in

 commit d79cdc8312689b39c6d83718c1c196af4b3cd18c
 Author: Daniel Vetter <daniel.vetter@ffwll.ch>
 Date:   Thu Aug 8 15:41:32 2013 +0200

    drm: no-op out GET_STATS ioctl

so clean up the leftovers as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>