drm/i915: Implement WaDisableVFclkgate.

This workaround supposedly fixes some hangs in the VF unit.

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171216001117.14232-1-rafael.antognolli@intel.com