drm/vc4: Wait for fences interruptibly in blocking mode.

We should allow SIGIO and things to interrupt us before we get to the
no-error stage of the commit process.  This code is effectively copied
from drm_atomic_helper_commit().

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621185002.28563-2-eric@anholt.net


Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>