drm/i915: Wrap port cancellation into a function

On reset and wedged path, we want to release the requests
that are tied to ports and then mark the ports to be unset.
Introduce a function for this.

v2: rebase
v3: drop local, keep GEM_BUG_ON (Michał, Chris)
v4: rebase

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170922124307.10914-3-mika.kuoppala@intel.com