drm/msm/phy/dsi_phy: Set pll to NULL in case initialization fails

We have if (!phy->pll) checks scattered through the driver and if
phy->pll is an error pointer, those checks will pass and bad things will
happen :(

Reviewed-by: Rob Clark <[e-mail]>
Signed-off-by: Sean Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/msm/dsi/phy/dsi_phy.c

···
 		goto fail;
 
 	phy->pll = msm_dsi_pll_init(pdev, phy->cfg->type, phy->id);
-	if (IS_ERR_OR_NULL(phy->pll))
+	if (IS_ERR_OR_NULL(phy->pll)) {
 		DRM_DEV_INFO(dev,
 			"%s: pll init failed: %ld, need separate pll clk driver\n",
 			__func__, PTR_ERR(phy->pll));
+		phy->pll = NULL;
+	}
 
 	dsi_phy_disable_resource(phy);