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>