gfio: encapsulate x- and y-offsets into graph library

gfio: encapsulate x- and y-offsets into graph library

Don't need to carry around drawing area dimensions alongside
the graphs, the size and xy-offsets of the graphs can be set
directly in the drawing area configure callback.

Signed-off-by: Stephen M. Cameron <stephenmcameron@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>