drm/i915: fix register naming

This name was added with the whitelisting of registers for building up OA
configs. It is contained in a range gen8 whitelist :

   addr >= RPM_CONFIG0.reg && addr <= NOA_CONFIG(8).reg

Hence why the name isn't used anywhere.

v2: Fix register name again RPC->RCP (Matthew)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110190845.32574-5-lionel.g.landwerlin@intel.com