Intel_extreme: improve i855 support.

https://github.com/druga/haiku-stuff/tree/master/intel_extreme
Rebased against current sources.

* The BIOS video mode sometimes reports a scaled mode instead of the
physical panel dimensions. Get the data from the VBT table as well, and
use it if the reported resolution is bigger.
* On first boot, force the panel native mode so the user doesn't have to
set it manually.
* Only allow a single head at a time on i855gm, as the card can't drive
both heads at the same time.
* Detect when a new requested mode is the same as the current one, and
skip modesetting in that case. Avoids screen flickering when changing
workspaces.
* Fix some cases of misdetecting which pipes to enable
Revert part of previous patch

Checking only the visible resolution and colorspace isn't enough to call
two modes equal, otherwise it is not possible to change the refresh rate
anymore.
Some style fixes.

More work is needed for this new file to completely follow the style
guidelines. Also added several FIXMEs and TODOs.

This doesn't seem to break anything on my machine however, and was
confirmed to help by several users, so we may as well use it.
Style fixes and allocation checks