drm/i915: Fix GCC 4.4 build issue with __intel_wait_for_register_fw

Move the BUILD_BUG_ONs for busy-wait duration outside the
_wait_for_atomic macro as discussed on the mailing list.

v2: Simplify the macro by omitting the ret__ local. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Fixes: 1d1a9774 ("drm/i915: Extend intel_wait_for_register_fw() with fast timeout")
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170418105211.7089-1-tvrtko.ursulin@linux.intel.com