drm/i915: Update ring freq for full gpu freq range

In __gen6_update_ring_freq, use the full range of
possible gpu frequencies from max_freq to min_freq.
The actual gpu frequency could be outside the range
from max_freq_softlimit to min_freq_softlimit due
to power/thermal constraints.

Signed-off-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>