drm/xe: Size GT device memory correctly

The current method of sizing GT device memory is not quite right.

Update the algorithm to use the relevant HW information and offsets
to set up the sizing correctly.

Update the stolen memory sizing to reflect the changes, and to be
GT specific.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>