drivers/video/fbdev/atyfb: Carve out framebuffer length fudging into a helper

The size of the framebuffer to be used needs to be fudged to
account for the different type of devices that are out there.
This captures what is required to do well, we'll reuse this
later.

This has no functional changes.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathias Krause <minipli@googlemail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: airlied@redhat.com
Cc: arnd@arndb.de
Cc: benh@kernel.crashing.org
Cc: dan.j.williams@intel.com
Cc: geert@linux-m68k.org
Cc: hch@lst.de
Cc: hmh@hmh.eng.br
Cc: jgross@suse.com
Cc: linux-fbdev@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: luto@amacapital.net
Cc: mpe@ellerman.id.au
Cc: mst@redhat.com
Cc: ralf@linux-mips.org
Cc: ross.zwisler@linux.intel.com
Cc: stefan.bader@canonical.com
Cc: syrjala@sci.fi
Cc: tj@kernel.org
Cc: toshi.kani@hp.com
Cc: ville.syrjala@linux.intel.com
Link: http://lkml.kernel.org/r/1435251019-32421-1-git-send-email-mcgrof@do-not-panic.com
Link: http://lkml.kernel.org/r/1436491499-3289-2-git-send-email-mcgrof@do-not-panic.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>