drm/xen-front: Drop pointless static qualifier in fb_destroy()

There is no need to have the 'struct drm_framebuffer *fb' variable
static since new value always be assigned before use it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1548504338-114487-1-git-send-email-yuehaibing@huawei.com