drm/cma-helper: Fix locking in drm_fb_cma_debugfs_show

This function takes two locks, both of them the wrong ones. This
wasn't an oversight from my fb locking rework since both patches
landed in parallel. We really only need fb_lock when walking that
list, since everything we can reach from that is refcounted properly
already.

v2: Drop unused dev spotted by 0day.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>