gma500: Make GTT pages uncached

Clean up the GTT code a bit, make the pages uncached and go via the proper
interfaces. This avoids any aliasing problems.

On the CPU side we need to access the pages via their true addresses not via
the GTT. This is fine for GEM created fb objects for X. For the kernel fb
when not in stolen RAM we are going to need to use vm_map_ram() and hope we
have enough virtual address space to steal.

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