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

commit cc1d3e032df53d83d0ca4d537d8eb67eb5b3e808 upstream.

Commit ba02fa37de80bea10d706f39f076dd848348320a 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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>