drm/i915: Remove unwarranted clamping for hsw/bdw

We always start off at an "efficient frequency" and can let the system
autotune from there, eliminating the need to clamp the available range.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190418205358.11450-1-chris@chris-wilson.co.uk