drm/i915/bdw: Use HSW formula for ring freq scaling

The current formula we use for HSW is not what is in current docs.
However, changing to the HSW formula on my HSW does not improve power
usage, and decreases performance by about 5% in limited xonotic testing.

For gen8, until we know otherwise, or run experiments, let's use
the HSW formula - which should be the same used in the Windows driver
(and thus help make an apples-applies comparison) on gen8.

v2: Use >= 8 instead of > 7 to be consistent with all other gen
checks.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>