drivers/gpu/drm/tilcdc: Makefile, only -Werror when no -W* in EXTRA_CFLAGS

  When make with EXTRA_CFLAGS=-W, it will report error.
  so give a check in Makefile.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>