video: slimport: Set/clear sp_tx_pd_mode in the power control function

sp_tx_pd_mode is the flag if tx power is down or not. So it should be
controlled in the power control function.

Change-Id: I394acbd6c6125dce8725aa81193b7be2abdf47b0
Signed-off-by: Devin Kim <dojip.kim@lge.com>
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>