stifb: detect cards in double buffer mode more reliably

Visualize-EG, Graffiti and A4450A graphics cards on PARISC can
be configured in double-buffer and standard mode, but the stifb
driver supports standard mode only.
This patch detects double-buffered cards more reliable.

It is a real bugfix for a very nasty problem for all parisc users which have
wrongly configured their graphic card.  The problem: The stifb graphics driver
will not detect that the card is wrongly configured and then nevertheless just
enables the graphics mode, which it shouldn't.  In the end, the user will see
no further updates / boot messages on the screen.

We had documented this problem already on our FAQ
(http://parisc-linux.org/faq/index.html#viseg

 "Why do I get corrupted graphics
with my Vis-EG/Graffiti/A4450A card?") but people still run into this problem.
 So having this fix in as early as possible can help us.

Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Cc: <stable@kernel.org>
Cc: Kyle McMartin <kyle@mcmartin.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>