drm/msm/sde: correct the hsync/vsync polarity cfg for dp

The current implementation uses active high for DP interface
hsync/vsync polarity config. Instead of using active high
always, this change configures the right polarity value
based on drm_display_mode flags.

Change-Id: Ief83d7faa2ace3867631c13b85d3c34fa05bcb27
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>