drm: More specific locking for get* ioctls

drm: More specific locking for get* ioctls

Motivated by the per-plane locking I've gone through all the get*
ioctls and reduced the locking to the bare minimum required.

v2: Rebase and make it compile ...

v3: Review from Sean:
- Simplify return handling in getplane_res.
- Add a comment to getplane_res that the plane list is invariant and
  can be walked locklessly.

v4: Actually git add.

Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>