DRM2: fix off-by-one overflow in ioctl processing

Call to the driver-specific ioctl used to process ioctl number
that will lead to the out-of-bounds access to the ioctl handler
array.

PR:		193367
Approved by:	kib
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275209 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f