Enable FtrPerCtxtPreemptionGranularityControl bit and select thread-
group as the default preemption level.

v2:
 * Remove register whitelisting (Rafael, Tony).
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: piotr.zdunowski@intel.com
Cc: michal.mrozek@intel.com
Cc: Tony Ye <tony.ye@intel.com>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Acked-by: Rafael Antognolli <rafael.antognolli@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Tony Ye <tony.ye@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304153144.10675-1-tvrtko.ursulin@linux.intel.com