i915: don't call drm_atomic_state_put on invalid pointer

The introduction of reference counting on the state structures caused
sanitize_watermarks() in i915 to break in the error handling case,
as pointed out by gcc -Wmaybe-uninitialized

drivers/gpu/drm/i915/intel_display.c: In function ‘intel_modeset_init’:
include/drm/drm_atomic.h:224:2: error: ‘state’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

This changes the function back to only drop the reference count
when it was successfully allocated first.

Fixes: 0853695c3ba4 ("drm: Add reference counting to drm_atomic_state")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161018151652.2690201-1-arnd@arndb.de

drivers/gpu/drm/i915/intel_display.c

@@ -16294,7 +16294,7 @@ retry:
 		 * BIOS-programmed watermarks untouched and hope for the best.
 		 */
 		WARN(true, "Could not determine valid watermarks for inherited state\n");
-		goto fail;
+		goto put_state;
 	}
 
 	/* Write calculated watermark values back */
@@ -16305,8 +16305,9 @@ retry:
 		dev_priv->display.optimize_watermarks(cs);
 	}
 
-fail:
+put_state:
 	drm_atomic_state_put(state);
+fail:
 	drm_modeset_drop_locks(&ctx);
 	drm_modeset_acquire_fini(&ctx);
 }