drm/msm/dp: add a missing unlock in dp_hpd_plug_handle()

When pm_runtime_resume_and_get() fails, unlock before returning.

Fixes: 5814b8bf ("drm/msm/dp: incorporate pm_runtime framework into DP driver")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/570395/
Link: https://lore.kernel.org/r/20231204171317.192427-1-harshit.m.mogalapalli@oracle.comSigned-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

drivers/gpu/drm/msm/dp/dp_display.c

@@ -564,6 +564,7 @@ static int dp_hpd_plug_handle(struct dp_display_private *dp, u32 data)
 	ret = pm_runtime_resume_and_get(&pdev->dev);
 	if (ret) {
 		DRM_ERROR("failed to pm_runtime_resume\n");
+		mutex_unlock(&dp->event_mutex);
 		return ret;
 	}