drm/i915/skl: add turbo support

Per latest PM programming guide.

v2: the wrong flavour of the function updating the ring frequency was
    called, leading to dead locks (Tvrtko)

v3: Add GEN6_RP_MEDIA_IS_GFX to RP_CONTROL (Imre, done by Damien)

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
[danvet: Fixup conflicts with Mika's forcewake refactor.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>