drm/i915: Move MOCS setup to intel_mocs.c

Hide the details of MOCS setup from i915_gem by moving both current calls
into one in intel_mocs_init.

Cc: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190713010940.17711-21-lucas.demarchi@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20190730180407.5993-6-lucas.demarchi@intel.com