drm/i915: Refactor out intel_context_init()

Prior to adding a third instance of intel_context_init() and extending
the information stored therewithin, refactor out the common assignments.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190121222117.23305-8-chris@chris-wilson.co.uk