drm/i915: don't enable autosuspend on platforms without RPM support

pm_runtime_{use,dont_use}_autosuspend() controls whether the device's
sysfs power/autosuspend_delay_ms file is writeable or returns -EIO on
access to user space. Since

commit 25b181b4
Author: Imre Deak <imre.deak@intel.com>
Date:   Thu Dec 17 13:44:56 2015 +0200

  drm/i915: get a permanent RPM reference on platforms w/o RPM support

this sysfs file is writeable also on platforms without RPM support, but
userspace (at least IGT) depends on this file being unchangable to
determine whether the device supports runtime PM at all. So restore the
old behavior.

This gets rid of igt/pm_rpm failures on old platforms without RPM
support, where the test should be skipped.

Testcase: igt/pm_rpm/basic-rte
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450371873-878-1-git-send-email-imre.deak@intel.com