video: da8xx-fb: pix clk and clk div handling cleanup

Based on original patch by: Afzal Mohammed <afzal@ti.com>

Use the new modedb field to store pix clk. Reorganize existing clock
divider functions with names now corresponding to what they do, add
common function prefix.

Fix existing panel modedb pixclock to be in ps instead of Hz. This
needed a change in the way clock divider is calculated. As modedb
pixclock information is now in ps, override on "var" pixclock over
modedb to var conversion is removed.

v2:
Changed pixel clock configuration to use KHZ2PICOS macro

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