drm/i915: Add a context open function

We'll be doing a bit more stuff with each file, so having our own open
function should make things clean.

This also allows us to easily add conditionals for stuff we don't want
to do when we don't have HW contexts.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>