video: slimport: Do not access the sp registers after power down

The burst i2c errors sometimes causes a watchdog reset. The i2c errors
are caused by accessing the sp registers after SP is power down. So
do not access the sp register after power down to avoid the watchdog reset.

Change-Id: If64b0bf6f437292f356fa165d4203d522c26cfe6
Signed-off-by: Devin Kim <dojip.kim@lge.com>