drm/fence: allow fence waiting to be interrupted by userspace

If userspace is running an synchronously atomic commit and interrupts the
atomic operation during fence_wait() it will hang until the timer expires,
so here we change the wait to be interruptible so it stop immediately when
userspace wants to quit.

Also adds the necessary error checking for fence_wait().

v2: Comment by Daniel Vetter
	- Add error checking for fence_wait()

v3: Rebase on top of new atomic noblocking support

v4: Comment by Maarten Lankhorst
	- remove 'swapped' bitfield as it was duplicating information

v5: Comments by Maarten Lankhorst
	- assign plane->state to plane_state if !intr
	- squash previous patch into this one

v6: Comment by Sean Paul
	- rename intr to pre_swap

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
[seanpaul fixed a couple checkpatch warnings and moved the preswap comment]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1473707291-14781-1-git-send-email-gustavo@padovan.org