drm: Only take cursor locks when the cursor plane exists

drm: Only take cursor locks when the cursor plane exists

I thought I've fixed this, but maybe not. Anyway, clearly broken, and
easy fix.

Cc: Tony Lindgren <tony@atomide.com>
Reported-by: Tony Lindgren <tony@atomide.com>
Fixes: b95ff0319a82 ("drm: Remove drm_modeset_(un)lock_crtc")
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407164817.28272-1-daniel.vetter@ffwll.ch