drm/i915: Skip logging impossible slices

Log up to sseu->max_slices instead basing on ARRAY_SIZE since to avoid
printing impossible and empty slices for a platform.

Also compact slice total and slice mask into one log line.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180321103228.32205-1-tvrtko.ursulin@linux.intel.com