drm/i915: Add lock on drm_helper_resume_force_mode

drm/i915: Add lock on drm_helper_resume_force_mode

BugLink: http://bugs.launchpad.net/bugs/981162

commit 927a2f119e8235238a2fc64871051b16c9bdae75 upstream.

i915_drm_thaw was not locking the mode_config lock when calling
drm_helper_resume_force_mode. When there were multiple wake sources,
this caused FDI training failure on SNB which in turn corrupted the
display.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Leann Ogasawara <leann.ogasawara@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>