drm/i915: Fix log type for RC6 debug messages

RC6 isn't really a KMS feature, so use the more proper DRIVER log type
for RC6 related debug messages.

CC: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1467216835-1086-1-git-send-email-imre.deak@intel.com