drm/i915: Force GPU Freq to lowest while suspending.

We might be leaving the GPU Frequency (and thus vnn) high during the suspend.
Force gt to move to lowest freq while suspending.

v2: Fixed typo in commit message (Deepak)

v3: Force gt to lowest freq in suspend_gt_powersave (Daniel)

v4: Add GPU min freq set _after_ we've cancelled the rps works (Daniel)

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>