drm/i915: don't trylock in the gpu reset code

Simply failing to reset the gpu because someone else might still hold
the mutex isn't a great idea - I see reliable silent reset failures.
And gpu reset simply needs to be reliable and Just Work.

"But ... the deadlocks!"

We already kick all processes waiting for the gpu before launching the
reset work item. New waiters need to check the wedging state anyway
and then bail out. If we have places that can deadlock, we simply need
to fix them.

"But ... testing!"

We have the gpu hangman, and if the current gpu load gem_exec_nop
isn't good enough to hit a specific case, we can add a new one.

"But ...  don't we return -EAGAIN for non-interruptible calls to
wait_seqno now?"

Yep, but this problem already exists in the current code. A follow up
patch will remedy this by returning -EIO for non-interruptible sleeps
if the gpu died and the low-level wait bails out with -EAGAIN.
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Tested-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -730,8 +730,7 @@ int i915_reset(struct drm_device *dev)
 	if (!i915_try_reset)
 		return 0;
 
-	if (!mutex_trylock(&dev->struct_mutex))
-		return -EBUSY;
+	mutex_lock(&dev->struct_mutex);
 
 	i915_gem_reset(dev);