drm/i915: Mark up potential allocation paths within i915_sw_fence as might_sleep

As kmalloc is allowed to block (if given the right flags), mark up the
two i915_sw_fence routines that may call kmalloc as potential sleeping
routines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171212180652.22061-1-chris@chris-wilson.co.uk