fbdev: vfb: add option for video mode

Make vfb a bit more flexible in sense what it can represent and allow
the end user to specify video mode parameters via newly introduced module
option "mode". Since it is test module it is still up to the end user
to make sure there is enough memory to satisfy video mode settings.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
[tomi.valkeinen@ti.com: constified vfb_default]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>