fbdev/omap2: use DEFINE_SPINLOCK() instead of spin_lock_init().

spinlock can be initialized automatically with DEFINE_SPINLOCK()
rather than explicitly calling spin_lock_init().

Signed-off-by: Jiahua Yu <yujiahua1@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616031713.24959-1-yujiahua1@huawei.com