intel/sanitize_gpu: deal with non page multiple buffer sizes

intel/sanitize_gpu: deal with non page multiple buffer sizes

We can only map at page aligned offsets. We got that wrong with buffer
size where (size % 4096) != 0 (anv has a WA buffer of 1024).

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>