drm/i915: don't schedule force_wake_timer at gen6_read

So far force_wake_timer was only used by gen6_gt_force_wake_put. Since
we always had balanced gen6_gt_force_wake_get/put calls, we could
guarantee balanced calls to intel_runtime_pm_get/put.

Commit 8232644c, "drm/i915: Convert
the forcewake worker into a timer func" started scheduling the
force_wake_timer at gen6_read, which resulted in an unbalanced
runtime_pm refcount.

So this commit just reverts to the old behavior until we can find a
proper way to used delayed force_wake from the register read/write
macros without leaving the runtime_pm refcounts unbalanced and without
runtime suspending the driver while forcewake is active.

Testcase: igt/pm_pc8/rte
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76544


Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>