drm/core: clean up references to drm_dev_unref()

This is a continuation of a previous commit ("drm: introduce drm_dev_{get/put}
functions") to replace all references to drm_dev_unref() in drm core files with
drm_dev_put().

Signed-off-by: Aishwarya Pant <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/20170926170400.GA7671@aishwarya

drivers/gpu/drm/drm_pci.c

···
 	drm_pci_agp_destroy(dev);
 	pci_disable_device(pdev);
 err_free:
-	drm_dev_unref(dev);
+	drm_dev_put(dev);
 	return ret;
 }
 EXPORT_SYMBOL(drm_get_pci_dev);

drivers/gpu/drm/drm_prime.c

···
 	/* drop the reference on the export fd holds */
 	drm_gem_object_put_unlocked(obj);
 
-	drm_dev_unref(dev);
+	drm_dev_put(dev);
 }
 EXPORT_SYMBOL(drm_gem_dmabuf_release);