video: da8xx-fb: fb_check_var enhancement

Check whether "struct fb_var_screeninfo" fields are sane, if not
update it to be within allowed limits.

If user sends down buggy "var" values, this will bring those within
allowable limits. And fb_set_par is not supposed to change "var"
values, fb_check_var has to ensure that values are proper.

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>