drm/doc: No more drm perf counters

Those all died with

commit 0111be42
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Fri Oct 4 14:53:41 2013 +0300

    drm: Kill drm perf counter leftovers

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>