drm/omap: cleanup fbdev init/free

omap_fbdev_init() and omap_fbdev_free() use priv->fbdev directly.
However, omap_fbdev_init() returns the fbdev, and omap_drv.c also
assigns the return value to priv->fbdev. This is slightly confusing.

Clean this up by removing the omap_fbdev_init() return value, as we
don't care whether fbdev init succeeded or not. Also change omap_drv.c
to call omap_fbdev_free() always, and omap_fbdev_free() does the check
if fbdev was initialized.

While at it, rename omap_fbdev_free() to omap_fbdev_fini() to better
match the "init" counterpart.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>