drm/exynos: vidi: use ctx->lock to protect struct vidi_context member variables related to memory alloc/free

In the Linux kernel, the following vulnerability has been resolved:

drm/exynos: vidi: use ctx->lock to protect struct vidi_context member variables related to memory alloc/free

Exynos Virtual Display driver performs memory alloc/free operations

without lock protection, which easily causes concurrency problem.

For example, use-after-free can occur in race scenario like this:

```

CPU0 CPU1 CPU2

---- ---- ----

vidi_connection_ioctl()

if (vidi->connection) // true

drm_edid = drm_edid_alloc(); // alloc drm_edid

...

ctx->raw_edid = drm_edid;

...

drm_mode_getconnector()

drm_helper_probe_single_connector_modes()

vidi_get_modes()

if (ctx->raw_edid) // true

drm_edid_dup(ctx->raw_edid);

if (!drm_edid) // false

...

vidi_connection_ioctl()

if (vidi->connection) // false

drm_edid_free(ctx->raw_edid); // free drm_edid

...

drm_edid_alloc(drm_edid->edid)

kmemdup(edid); // UAF!!

...

```

To prevent these vulns, at least in vidi_context, member variables related

to memory alloc/free should be protected with ctx->lock.