drm: simplify kcalloc() call to kzalloc().

Calls to kcalloc() for a single element can be simplified to calls to
kzalloc().

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Dave Airlie <airlied@linux.ie>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>