video: da8xx-fb: use modedb helper to update var

modedb structure is now used to store panel information, run modedb
helper over it for initial update of "var" information instead of
equating each fields.

While at it, remove redundant update of bits_per_pixel.

Note: pixclock is overridden with proper value using an existing code
as currently modedb is having it in Hz instead of ps, this would be
fixed in a later change and this overide would be removed.

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>