OMAPDSS: VENC: fix NULL pointer dereference in DSS2 VENC sysfs debug attr on OMAP4

Commit ba02fa37 disabled the
venc driver registration on OMAP4. Since the driver never gets
probed/initialised your get a dereferenceed NULL pointer if you
try to get info from /sys/kernel/debug/omapdss/venc

Return info message about disabled venc if venc_dump_regs() gets called.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>