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: Ville Syrjälä <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/drm_rect.c

···
  * 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.
  */