drm/i915: Store the HW min frequency as min_freq

this leaves a temporarily awkward min_delay (the soft limit) with the
new min_freq (the hardware limit). It's fixed in the next patch.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>