drm/i915: add msecs_to_jiffies_timeout to guarantee minimum duration

We need this to avoid premature timeouts whenever scheduling a timeout
based on the current jiffies value. For an explanation see [1].
The following patches will take the helper into use.

Once the more generic solution proposed in the thread at [1] is accepted
this patch can be reverted while keeping the follow-up patches.

[1] http://marc.info/?l=linux-kernel&m=136854294730957&w=2

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