drm/gma500: Remove dev_priv branch from unload function

The value of dev_priv in psb_driver_unload() is always non-zero. Remove
the respective test.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210920141051.30988-5-tzimmermann@suse.de