video: da8xx-fb: report correct pixclock

Update "var" pixclock with the value that is configurable in hardware.
This lets user know the actual pixclock.

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