drm/i915: make rc6 in sysfs functions conditional

Commit 0136db58

 merges rc6 information
into the power group. However, when compiled with CONFIG_PM not set,
modprobing i915 would taint since power_group_name is defined as NULL.

This patch makes these rc6 in sysfs functions conditional upon the
definition of the CONFIG_PM macro to avoid the above-mentioned problem.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=45181
Cc: stable@vger.kernel.org
Tested-by: Kris Karas <bugs-a12@moonlit-rail.com>
Signed-off-by: Hunt Xu <mhuntxu@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>