gma500: Ensure the frame buffer has a linear virtual mapping

We need this for the framebuffer in order to ensure that the kernel
framebuffer layer can handle it when using KMS. Except for the base
framebuffer this isn't a concern.

Add an npage field to the gtt as too many copies of the page calculation
are getting spread around the code.

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