tridentfb: fix console freeze when switching from X11

This patch fixes two problems when acceleration is enabled:

 - console switch from the Xorg locks up the computer
   because the Xorg code locks some registers and disables
   the mmio mode, so reenable these in the
   tridentfb_set_par() and enable_mmio()

 - blacklist the Image975 chipset from setting PCI burst
   mode. This helps with random lock ups of the
   framebuffer on this chip. The same fix is probably
   needed for the Xorg as well.

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>