drm/omap: fix missing unref to fb's buf object

omap_fbdev_create() takes a reference to the fb's gem object with
omap_gem_get_paddr(). However, it never releases it with
omap_gem_put_paddr().

This patch adds the missing omap_gem_put_paddr() to omap_fbdev_free().

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