drm/i915/cnl: Setup PAT Index.

Different from previous platforms, on CNL+ there's separated
registers for separated indexes.

v2: Remove comments regarding uncertainty around the table.
v3: Remove extra line (by Ben)

Cc: Clint Taylor <clinton.a.taylor@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170815232539.3562-1-rodrigo.vivi@intel.com