video: da8xx-fb fixing incorrect porch mappings

The driver was mapping the wrong fbdev margins to the
front porch / back porch for both vertical and horizontal
timings.

This patch corrects it so that:

hfp = right margin
hbp = left margin
vbp = upper margin
vfp = lower margin

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