Correct signedness bug in drm_modeset_ctl

drm_modeset_ctl() takes a signed in from userland, does a boundscheck,
and then uses it to index into a structure and write to it. The
boundscheck only checks upper bound, and never checks for nagative
values. If the int coming from userland is negative [after conversion]
it will bypass the boundscheck, perform a negative index into an array
and write to it, causing memory corruption.

Note that this is in the "old" drm driver; this issue does not exist
in drm2.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: cem
MFC after: 1 day
Sponsored by: The FreeBSD Foundation