drm/fb-helper: Clarify drm_fb_helper_restore_fbdev_mode*()

As of commit 5ea1f752 ("drm: add
drm_fb_helper_restore_fbdev_mode_unlocked()"),
drm_fb_helper_restore_fbdev_mode() is no longer public, and drivers
should call drm_fb_helper_restore_fbdev_mode_unlocked() from their
->lastclose callbacks instead.

Update the documentation to reflect this, and absorb the one liner
drm_fb_helper_restore_fbdev_mode() into its single caller.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>