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