drm/vmwgfx: fix lock breakage

After:

commit d059f652
Author:     Daniel Vetter <daniel.vetter@ffwll.ch>
AuthorDate: Fri Jul 25 18:07:40 2014 +0200

    drm: Handle legacy per-crtc locking with full acquire ctx

drm_mode_cursor_common() was switched to use drm_modeset_(un)lock_crtc()
which uses full aquire ctx.  So dropping/reaquiring the lock via
drm_modeset_(un)lock() directly isn't the right thing to do, as lockdep
kindly points out.

The 'FIXME's about sorting out whether vmwgfx *really* needs to lock-all
for cursor updates still apply.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Tested-by: Thomas Hellstrom <thellstrom@vmware.com>