nvidiafb: ensure that CRTC registers are accessible

- Ensure that CRTC registers are accessible by unlocking them on set_par(),
  otherwise i2c reading will fail.

- The function nvidia_vga_protect(), does not protect the VGA registers, but
  turns off the screen.  Rename it to nvidia_screen_off().

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>