drm/i915: Don't use GEN6_RC_VIDEO_FREQ on gen10+

GEN6_RC_VIDEO_FREQ is deprecated for >= gen10;
don't try to program it.

v2: Use IS_GEN9() instead of INTEL_GEN() and remove comment (Rodrigo)

Signed-off-by: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171117080146.20150-1-david.weinehall@linux.intel.com