drm/atomic: Remove unneeded null check for private objects

It can be seen in drm_atomic_get_private_obj_state() that
ptr will never be NULL, so skip the check for that case.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170927083532.5756-1-maarten.lankhorst@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>