agp/intel: make intel-gtt.c into a real source file

Now that the disentangling is complete, stop including intel-gtt.c
from intel-agp.c.

The linux build system _really_ doesn't allow .c source files with the
same name as the module. It fails with the following message when trying
to build such a bugger:

make[3]: Circular drivers/char/agp/intel-agp.o <- drivers/char/agp/intel-agp.o dependency dropped.

Instead of renameing intel-agp.c I've simply created a new module out
of intel-gtt.c. Renaming intel-agp.ko to something else is not an option
for it will surely kill someones boot process.

This also paves the way to use the gtt code without loading the agp
driver.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>