drm/i915/preempt: Fix WaEnablePreemptionGranularityControlByUMD

The WA applies to all production Gen9 and requires both enabling and
whitelisting of the per-context preemption control register.

v2: Extend to Cannonlake.

Signed-off-by: Jeff McGee <jeff.mcgee@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171003203453.15692-1-chris@chris-wilson.co.uk