drm: fix warning in drm_fops.c

drivers/char/drm/drm_fops.c: In function 'drm_setup':
drivers/char/drm/drm_fops.c:60: warning: comparison of distinct pointer types lacks a cast

Unfortunately PAGE_SIZE has different types on different architectures.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>