drm/i915/bxt: Broxton uses the same GMS values as Skylake

v2: Rebase on top of the early-quirks rework from Ville.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (v1)
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>