drm/msm/mdp5: Return error code in mdp5_mixer_release when deadlock is detected

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

drm/msm/mdp5: Return error code in mdp5_mixer_release when deadlock is detected

There is a possibility for mdp5_get_global_state to return

-EDEADLK when acquiring the modeset lock, but currently global_state in

mdp5_mixer_release doesn't check for if an error is returned.

To avoid a NULL dereference error, let's have mdp5_mixer_release

check if an error is returned and propagate that error.

Patchwork: https://patchwork.freedesktop.org/patch/485181/