drm/exynos: removed unnecessary variable.

Signed-off-by: NInki Dae <inki.dae@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_buf.c

@@ -29,8 +29,6 @@
 #include "exynos_drm_drv.h"
 #include "exynos_drm_buf.h"
 
-static DEFINE_MUTEX(exynos_drm_buf_lock);
-
 static int lowlevel_buffer_allocate(struct drm_device *dev,
 		struct exynos_drm_buf_entry *entry)
 {