amlogic: drm: meson_gem: Fix a deadlock issue

amlogic: drm: meson_gem: Fix a deadlock issue

After enabled the lock debug config, the below message is displayed.

[   20.897676@5] =============================================
[   20.899234@5] [ INFO: possible recursive locking detected ]
[   20.904782@5] 4.9.241-TIZEN-amlogic-kvim+ #6 Not tainted
[   20.910067@5] ---------------------------------------------
[   20.915614@5] enlightenment/579 is trying to acquire lock:
[   20.921074@5]  (&dev->struct_mutex){+.+...}, at: [<ffffff80096db7d8>] drm_gem_object_unreference_unlocked+0x20/0x140
[   20.931561@5]
[   20.931561@5] but task is already holding lock:
[   20.937714@5]  (&dev->struct_mutex){+.+...}, at: [<ffffff8009c8a744>] am_meson_gem_cpu_fini_ioctl+0x34/0xe8
[   20.947421@5]
[   20.947421@5] other info that might help us debug this:
[   20.954267@5]  Possible unsafe locking scenario:
[   20.954267@5]
[   20.960507@5]        CPU0
[   20.963107@5]        ----
[   20.965707@5]   lock(&dev->struct_mutex);
[   20.969693@5]   lock(&dev->struct_mutex);
[   20.973680@5]
[   20.973680@5]  *** DEADLOCK ***
[   20.973680@5]
[   20.980094@5]  May be due to missing lock nesting notation
[   20.980094@5]
[   20.987201@5] 1 lock held by enlightenment/579:
[   20.991707@5]  #0:  (&dev->struct_mutex){+.+...}, at: [<ffffff8009c8a744>] am_meson_gem_cpu_fini_ioctl+0x34/0xe8
[   21.001847@5]
[   21.001847@5] stack backtrace:
[   21.006529@5] CPU: 5 PID: 579 Comm: enlightenment Not tainted 4.9.241-TIZEN-amlogic-kvim+ #6
[   21.014934@5] Hardware name: Khadas VIM3 (DT)
[   21.019268@5] Call trace:
[   21.021871@5] [<ffffff800908b9e8>] dump_backtrace+0x0/0x268
[   21.027415@5] [<ffffff800908bc78>] show_stack+0x28/0x38
[   21.032616@5] [<ffffff80095b46d4>] dump_stack+0xe0/0x12c
[   21.037903@5] [<ffffff8009122df4>] __lock_acquire+0x1064/0x1b28
[   21.043795@5] [<ffffff8009123e80>] lock_acquire+0xe8/0x288
[   21.049255@5] [<ffffff80096db808>] drm_gem_object_unreference_unlocked+0x50/0x140
[   21.056709@5] [<ffffff8009c8a7b8>] am_meson_gem_cpu_fini_ioctl+0xa8/0xe8
[   21.063382@5] [<ffffff80096dcbe4>] drm_ioctl+0x204/0x4a8
[   21.068670@5] [<ffffff80092a1a74>] do_vfs_ioctl+0x674/0x8e0
[   21.074215@5] [<ffffff80092a1d74>] SyS_ioctl+0x94/0xa8
[   21.079328@5] [<ffffff8009083a1c>] __sys_trace_return+0x0/0x4
[   21.166643@2] input: Virtual Key Device as /devices/virtual/input/input6

Change-Id: Iadb1df81b88b288f2d6841520ce55c2ee47cedef
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>