drm/i915: cache number of MOCS entries

Instead of checking the gen number every time we need to know the max
number of entries, just save it into the table struct so we don't need
extra branches throughout the code. This will be useful for Ice Lake
that has 64 rather than 62 defined entries. Ice Lake changes will be
added in a follow up.

v2: make size and n_entries `unsigned int` and introduce changes as a
    pre-work for the Ice Lake changes (Tvrtko)

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
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/20190124000604.18861-7-lucas.demarchi@intel.com