intel_ips -- ensure we do not enable gpu turbo mode without driver linkage

Both when polling the current turbo status (in poll_turbo_status mode)
and when handling thermal events (in ips_irq_handler) the current status
of GPU turbo is updated to match the hardware status.  However if during
driver initialisation we were unable aquire linkage to the i915 driver
enabling GPU turbo will lead to an oops on the first attempt to determine
GPU busy status.

Ensure that we do not enable GPU turbo unless we have driver linkage.

BugLink: http://bugs.launchpad.net/bugs/632430


Cc: stable@kernel.org
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>