video: da8xx-fb: improve readability of code

Change the lcd_disable_raster funtion from using a bool to an enum
as the function is very confusing with the current api.  This helps
make it clearer what the parameter is really doing.

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