drm/i915: avoid premature timeouts in __wait_seqno()

At the moment wait_event_timeout/wait_event_interruptible_timeout may
time out 1 jiffy too early, as the calculated expiry time is 1 less than
needed. Besides timing out too early this also means that the
calculation of the remaining time will be incorrect and we will pass a
non-zero remaining time to user space in case of a time out. This is one
reason for the following bugzilla report:

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=64270



Signed-off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>