gma500: Add support for inserting and removing pages from the GART

There are two chunks of code we need to do this. The first one is the code
to insert and remove the pages from the GART, the second is the code to build
page table lists from the GEM object. Surprisingly this latter one doesn't seem
to have a nice GEM helper.

While we are at it we can begin dismantling the semi redundant struct pg,
and finish pruning out the old now unused gtt code as well as the last bits
of helper glue from the old driver base.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>