drm/i915: Re-enable self-refresh

A broken implementation of is_pot() prevented the detection of when a
singular pipe was enabled. Eric Anholt pointed out the existence of
is_power_of_2() so use that instead of our broken code!

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=35402


Signed-off-by: Yuanhan Liu <yuanhan.liu@intel.com>
Tested-by:  <xunx.fang@intel.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>