drm/etnaviv: Fix missing mutex_destroy()

Currently, the calling of mutex_destroy() is ignored on error handling
code path. It is safe for now, since mutex_destroy() actually does
nothing in non-debug builds. But the mutex_destroy() is used to mark
the mutex uninitialized on debug builds, and any subsequent use of the
mutex is forbidden.

It also could lead to problems if mutex_destroy() gets extended, add
missing mutex_destroy() to eliminate potential concerns.

Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>