drm: fix drm_mode_getconnector() locking imbalance regression

Regression in commit 2caa80e7


Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Feb 22 11:38:36 2015 +0100

    drm: Fix deadlock due to getconnector locking changes

If the drm_connector_find() call returns NULL, we should no longer
call drm_modeset_unlock() to avoid locking imbalance.

Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>