drm/i915: Drop mutex around clearing error state

drm/i915: Drop mutex around clearing error state

The error state itself is guarded by a spinlock (admittedly even that is
overkill for a single pointer!) and doesn't require us to take the
struct_mutex in the debugfs/sysfs interface. Removing the struct_mutex
removes one more potential blockage when trying to debug a deadlock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20160901205510.31307-1-chris@chris-wilson.co.uk
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com