drm: Avoid NULL dereference for DRM_LEGACY debug message

smatch warns:

	drivers/gpu/drm/drm_lock.c:188 drm_legacy_lock() warn:
	variable dereferenced before check 'master->lock.hw_lock' (see line 177)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161127170910.29106-2-chris@chris-wilson.co.uk