drm: i810/i830: fix locked ioctl variant

The i810 and i830 device drivers may replace their file operations
on an open file descriptor. My previous patch to move the BKL
out of the common DRM code into these drivers only caught the
default file operations, not the ones that actually end up being
used.

Found while trying to come up with a way to kill the BKL for
good in these drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>