drm/i915: Widen return value for reservation_object_wait_timeout_rcu to long.

This fixes a spurious warning from an integer overflow on 64-bits systems.
The function may return MAX_SCHEDULE_TIMEOUT which gets truncated to -1.

Explicitly handling this by casting to lret fixes it.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reported-and-tested-by: Joseph Yasi <joe.yasi@gmail.com>
Tested-by: Andreas Reis <andreas.reis@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: drm-intel-fixes@lists.freedesktop.org
Fixes: 3c28ff22 ("i915: wait for fence in prepare_plane_fb")
Link: http://patchwork.freedesktop.org/patch/msgid/5666EEC8.2000403@linux.intel.com