firmware: sysfb: Fix reference count of sysfb parent device

Retrieving the system framebuffer's parent device in sysfb_init()
increments the parent device's reference count. Hence release the
reference before leaving the init function.

Adding the sysfb platform device acquires and additional reference
for the parent. This keeps the parent device around while the system
framebuffer is in use.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 9eac534d

 ("firmware/sysfb: Set firmware-framebuffer parent device")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Sui Jingfeng <suijingfeng@loongson.cn>
Cc: <stable@vger.kernel.org> # v6.9+
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240625081818.15696-1-tzimmermann@suse.de