drm: Fix DP_TEST_COUNT_MASK

By Vesa's DP 1.2 Spec this counter has 4 bits [3:0].

This mask is wrong since when the counter was introduced by myself
on commit ad9dc91b
Author: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Date:   Tue Sep 16 19:18:12 2014 -0400

    drm/i915: Fix Sink CRC

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>