video: fbdev: pmag-ba-fb: Fix the lower margin size

According to the board specification[1] the width of the vertical sync
front porch is 12 pixels or the same as the width of the horizontal sync
front porch.  This in turn means the size of the lower margin is 0,
because the vertical sync starts as soon as the start of the horizontal
sync terminates the last line.

References:

[1] "PMAG-BA TURBOchannel Color Frame Buffer Functional Specification",
    Revision 1.2, Workstation Systems Engineering, Digital Equipment
    Corporation, August 27, 1990, Table 3-5: "Video Timing"

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>