cirrusfb: various Laguna fixes

cirrusfb: various Laguna fixes

- The Laguna GD5465 (AGP) has one register more than non-AGP chips.
  Recognize the AGP version and write a tile control register only on the
  AGP version.  Tested only on an AGP card.

- Bump up RAMDAC frequencies after X11 code.  This allow to drive a flat
  panel resolution 1680x1050 at 16bpp from the 4MB card.

- Fix screen start address overflow bits on Laguna cards (CRT1D
  register).

- Fix exit path in the cirrusfb_pci_register() in case of error.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>