drm/i915: Don't increase the GPU frequency from the delayed VLV rps timer

There's little point in increasing the GPU frequency from the delayed
rps work on VLV. Now when the GPU is idle, the GPU frequency actually
keeps dropping gradually until it hits the minimum, whereas previously
it just ping-ponged constantly between RPe and RPe-1.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>