drm/i915: Fix module unloading with DRM_I915_UMS=n

Oops, makes testing early boot failures in i915.ko a bit more pain, so
let's fix it.

v2: We already have a bit of static storage to track this (Chris).
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.c

@@ -1022,6 +1022,11 @@ static int __init i915_init(void)
 
 static void __exit i915_exit(void)
 {
+#ifndef CONFIG_DRM_I915_UMS
+	if (!(driver.driver_features & DRIVER_MODESET))
+		return; /* Never loaded a driver. */
+#endif
+
 	drm_pci_exit(&driver, &i915_pci_driver);
 }