drm/sysfs: Grab lock for edid/modes_show

drm/sysfs: Grab lock for edid/modes_show

We chase pointers/lists without taking the locks protecting them,
which isn't that good.

Fix it.

v2: Actually unlock properly, spotted by Julia.

v3: Put the label _before_ the mutex_unlock (Emil)

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Link: http://patchwork.freedesktop.org/patch/msgid/1443783662-23066-1-git-send-email-daniel.vetter@ffwll.ch
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>