drm/dp/mst: Handle errors from drm_atomic_get_private_obj_state() correctly

drm/dp/mst: Handle errors from drm_atomic_get_private_obj_state() correctly

On failure drm_atomic_get_private_obj_state() returns and error
pointer instead of NULL. Adjust the checks in the callers to match.

Cc: stable@vger.kernel.org
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Fixes: 0beaadf6f97e ("drm/dp: Add DP MST helpers to atomically find and release vcpi slots")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170712155102.26276-1-ville.syrjala@linux.intel.com
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>