drm/i915: drop the struct_mutex when wedged or trying to reset

We grab the struct_mutex in intel_crtc_page_flip, but if we are wedged
or a reset is in progress we bail early but never seem to actually
release the lock.

Fixes: 7f1847eb

 ("drm/i915: Simplify checking of GPU reset_counter in display pageflips")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161128103648.9235-1-matthew.auld@intel.com
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <stable@vger.kernel.org> # v4.7+
(cherry picked from commit ddbb271a

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>