drm/msm/msm_gem_shrinker: Fix descriptions for 'drm_device'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/msm/msm_gem_shrinker.c:108: warning: Function parameter or member 'dev' not described in 'msm_gem_shrinker_init'
 drivers/gpu/drm/msm/msm_gem_shrinker.c:108: warning: Excess function parameter 'dev_priv' description in 'msm_gem_shrinker_init'
 drivers/gpu/drm/msm/msm_gem_shrinker.c:126: warning: Function parameter or member 'dev' not described in 'msm_gem_shrinker_cleanup'
 drivers/gpu/drm/msm/msm_gem_shrinker.c:126: warning: Excess function parameter 'dev_priv' description in 'msm_gem_shrinker_cleanup'

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: linux-arm-msm@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: freedreno@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>