drm/i915/kvmgt: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace the following form:

sizeof(*sparse) + (nr_areas * sizeof(*sparse->areas)

with:

struct_size(sparse, areas, sparse->nr_areas)

and so on...

Also, notice that variable size is unnecessary, hence it is removed.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>