video: da8xx-fb: reorganize panel detection

Move panel detection to a separate function, this helps in readability
as well as makes DT support cleaner.

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>