drm/i915: rc6 in sysfs

Merge rc6 information into the power group for our device. Until now the
i915 driver has not had any sysfs entries (aside from the connector
stuff enabled by drm core). Since it seems like we're likely to have
more in the future I created a new file for sysfs stubs, as well as the
rc6 sysfs functions which don't really belong elsewhere (perhaps
i915_suspend, but most of the stuff is in intel_display,c).

displays rc6 modes enabled (as a hex mask):
cat /sys/class/drm/card0/power/rc6_enable

displays #ms GPU has been in rc6 since boot:
cat /sys/class/drm/card0/power/rc6_residency_ms

displays #ms GPU has been in deep rc6 since boot:
cat /sys/class/drm/card0/power/rc6p_residency_ms

displays #ms GPU has been in deepest rc6 since boot:
cat /sys/class/drm/card0/power/rc6pp_residency_ms

Important note: I've seen on SNB that even when RC6 is *not* enabled the
rc6 register seems to have a random value in it. I can only guess at the
reason reason for this. Those writing tools that utilize this value need
to be careful and probably want to scrutinize the value very carefully.

v2: use common rc6 residency units to milliseconds for the other RC6 types

v3: don't create sysfs files for GEN <= 5
add a rc6_enable to show a mask of enabled rc6 types
use unmerge instead of remove for sysfs group
squash intel_enable_rc6() extraction into this patch

v4: rename sysfs files (Chris)

CC: Chris Wilson <chris@chris-wilson.co.uk>
CC: Daniel Vetter <daniel.vetter@ffwll.ch>f
CC: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: squash in the 64bit division fix by Chris Wilson.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>