viafb: complete support for VX800/VX855 accelerated framebuffer

This patch is a painful merge of change
a90bab567ece3e915d0ccd55ab00c9bb333fa8c0 (viafb: Add support for 2D
accelerated framebuffer on VX800/VX855) in the OLPC tree, originally by
Harald Welte.  Harald's changelog read:

	The VX800/VX820 and the VX855/VX875 chipsets have a different 2D
    	acceleration engine called "M1".  The M1 engine has some subtle
    	(and some not-so-subtle) differences to the previous engines, so
    	support for accelerated framebuffer on those chipsets was disabled
    	so far.

This merge tries to preserve Harald's changes in the framework of the
much-changed 2.6.34 viafb code.

Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: ScottFang@viatech.com.cn
Cc: JosephChan@via.com.tw
Signed-off-by: Jonathan Corbet <corbet@lwn.net>