drm/i915/selftests: Drop unnecessary struct_mutex around i915_reset()

Since we no longer need to hold struct_mutex to perform a global device
reset, don't do so for igt_reset_wedge().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190215102732.15520-2-chris@chris-wilson.co.uk