drm/irq: Document return values more consistently

Some of the functions are documented inconsistently. Add Returns:
sections where missing and use consistent style to describe the return
value.

Signed-off-by: Thierry Reding <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/drm_irq.c

···
  * drm_vblank_enable - enable the vblank interrupt on a CRTC
  * @dev: DRM device
  * @pipe: CRTC index
+ *
+ * Returns:
+ * Zero on success or a negative error code on failure.
  */
 static int drm_vblank_enable(struct drm_device *dev, unsigned int pipe)
 {
···
  * This is the legacy version of drm_crtc_vblank_get().
  *
  * Returns:
- * Zero on success, nonzero on failure.
+ * Zero on success or a negative error code on failure.
  */
 int drm_vblank_get(struct drm_device *dev, unsigned int pipe)
 {
···
  * This is the native kms version of drm_vblank_get().
  *
  * Returns:
- * Zero on success, nonzero on failure.
+ * Zero on success or a negative error code on failure.
  */
 int drm_crtc_vblank_get(struct drm_crtc *crtc)
 {