drm/i915: Make some RPS functions static

They are not used anywhere else. Also, fix a small typo in a comment.

No functional changes.

Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1503532705-3692-1-git-send-email-oscar.mateo@intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/1503532705-3692-1-git-send-email-oscar.mateo@intel.com