video: da8xx-fb: fix the polarities of the hsync/vsync pulse

The polarities were being set to active low when fbdev was requesting active
high.  This patch reverses it so that what is set into the LCD controller is
correct.

Signed-off-by: Darren Etheridge <detheridge@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>