drm/i915: cleanup a min_t() cast

The lower layers of sysfs will not allow an "offset" of more than
GEN7_L3LOG_SIZE and also l3_access_valid() caps it a second time.  But
it's a little easier to audit if we don't have to worry that the
subtraction will result in negative values.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>