drm/i915: Extend intel_wait_for_register_fw() with fast timeout

drm/i915: Extend intel_wait_for_register_fw() with fast timeout

In some cases we may want to spend more time in atomic wait than
hardcoded 2us. Let's add additional fast timeout parameter to allow
flexible configuration of atomic timeout before switching into heavy wait.
Add also possibility to return registry value to avoid extra read.

v2: use explicit fast timeout (Tvrtko/Chris)
    allow returning register value (Chris)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407160145.181328-1-michal.wajdeczko@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>