drm: use memchr_inv()

Use memchr_inv() to check the specified memory region is filled with zero.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Dave Airlie <airlied@redhat.com>