drm/i915: initialize unused MOCS entries to PTE

Instead of initializing them to uncached, let's set them to PTE for
kernel tracking. While at it do some minor adjustments to comments and
coding style.

From Chris: "What it does mean is that the buffer contents are consistent
with our cache tracking; and for userspace the results were always
undefined. So we should at least be able to guarantee that the data
written by userspace from the CPU is visible. After that, your caches
are on your own".

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tomasz Lis <tomasz.lis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190124000604.18861-2-lucas.demarchi@intel.com