drm/rect: Fix drm_rect_rotation_inv() docs

An overeager sed has corrupted the drm_rect_rotation_inv()
documentation. Fix it up.

Looks like it wasn't entirely correct before the sed fail
either. We were missing _rect_ from the function names, which
also explains why the sed hit these by accident.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180426141631.15798-1-ville.syrjala@linux.intel.comReviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_rect.c

@@ -373,8 +373,8 @@ EXPORT_SYMBOL(drm_rect_rotate);
  * them when doing a rotatation and its inverse.
  * That is, if you do ::
  *
- *     DRM_MODE_PROP_ROTATE(&r, width, height, rotation);
- *     DRM_MODE_ROTATE_inv(&r, width, height, rotation);
+ *     drm_rect_rotate(&r, width, height, rotation);
+ *     drm_rect_rotate_inv(&r, width, height, rotation);
  *
  * you will always get back the original rectangle.
  */