drm/msm/dp: remove an unnecessary NULL check in dp_power sub module

This is a change to remove an unnecessary NULL check in
dp_power_pinctrl_set function of dp_power sub module.

Change-Id: I21969826337ccace04f1d066974c089f34d068d9
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>