drm: fix null-ptr-deref in drm_dev_init_release()

I got a null-ptr-deref report:

[drm:drm_dev_init [drm]] *ERROR* Cannot allocate anonymous inode: -12
==================================================================
BUG: KASAN: null-ptr-deref in iput+0x3c/0x4a0
...
Call Trace:
 dump_stack_lvl+0x6c/0x8b
 kasan_report.cold+0x64/0xdb
 __asan_load8+0x69/0x90
 iput+0x3c/0x4a0
 drm_dev_init_release+0x39/0xb0 [drm]
 drm_managed_release+0x158/0x2d0 [drm]
 drm_dev_init+0x3a7/0x4c0 [drm]
 __devm_drm_dev_alloc+0x55/0xd0 [drm]
 mi0283qt_probe+0x8a/0x2b5 [mi0283qt]
 spi_probe+0xeb/0x130
...
 entry_SYSCALL_64_after_hwframe+0x44/0xae

If drm_fs_inode_new() fails in drm_dev_init(), dev->anon_inode will point
to PTR_ERR(...) instead of NULL. This will result in null-ptr-deref when
drm_fs_inode_free(dev->anon_inode) is called.

drm_dev_init()
	drm_fs_inode_new() // fail, dev->anon_inode = PTR_ERR(...)
	drm_managed_release()
		drm_dev_init_release()
			drm_fs_inode_free() // access non-existent anon_inode

Define a temp variable and assign it to dev->anon_inode if the temp
variable is not PTR_ERR.

Fixes: 2cbf7fc6 ("drm: Use drmm_ for drm_dev_init cleanup")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211013114139.4042207-1-wanghai38@huawei.com