libpciaccess MTRR splitting

[Francisco Jerez]

Hi.

Since I switched to libpciaccess, I've been seeing errors like:

> error setting MTRR (base = 0x14200000, size = 0x00800000, type = 1) Invalid argument (22)

It's because the GPU I'm using (a siliconmotion SM720) has a register
area at the beginning of the PCI BAR which shouldn't be set to WC, and
the remaining memory (the actual framebuffer) isn't aligned at a size
multiple.

I've been running a modified version of libpciaccess (diff attached)
that splits the memory range on properly aligned chunks before setting
the MTRRs.

If it turns out to be a reasonable approach, I'm willing to document
it more thoroughly and format it for git-am.

I've only implemented it for the linux_sysfs backend, but the logic at
pci_mapping_split is somewhat generic and another source file might be
better suited for it.

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