drm/msm: Upgrade gxpd checks to IS_ERR_OR_NULL

dev_pm_domain_attach_by_name() can return NULL, so we should check for
that case when we're about to dereference gxpd.

Fixes: 9325d426 ("drm/msm/gpu: Attach to the GPU GX power domain")
Cc: Jordan Crouse <jcrouse@codeaurora.org>
Cc: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jordan Crouse <jcrouse@codeauorora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190515170104.155525-1-sean@poorly.run

drivers/gpu/drm/msm/adreno/a6xx_gmu.c

@@ -747,7 +747,7 @@ int a6xx_gmu_resume(struct a6xx_gpu *a6xx_gpu)
 	 * will make sure that the refcounting is correct in case we need to
 	 * bring down the GX after a GMU failure
 	 */
-	if (!IS_ERR(gmu->gxpd))
+	if (!IS_ERR_OR_NULL(gmu->gxpd))
 		pm_runtime_get(gmu->gxpd);
 
 out:
@@ -863,7 +863,7 @@ int a6xx_gmu_stop(struct a6xx_gpu *a6xx_gpu)
 	 * domain. Usually the GMU does this but only if the shutdown sequence
 	 * was successful
 	 */
-	if (!IS_ERR(gmu->gxpd))
+	if (!IS_ERR_OR_NULL(gmu->gxpd))
 		pm_runtime_put_sync(gmu->gxpd);
 
 	clk_bulk_disable_unprepare(gmu->nr_clocks, gmu->clocks);
@@ -1234,7 +1234,7 @@ void a6xx_gmu_remove(struct a6xx_gpu *a6xx_gpu)
 
 	pm_runtime_disable(gmu->dev);
 
-	if (!IS_ERR(gmu->gxpd)) {
+	if (!IS_ERR_OR_NULL(gmu->gxpd)) {
 		pm_runtime_disable(gmu->gxpd);
 		dev_pm_domain_detach(gmu->gxpd, false);
 	}