drm/i915: Split struct intel_context definition to its own header

This complex struct pulling in half the driver deserves its own
isolation in preparation for intel_context becoming an outright
complicated class of its own.

In order to split this beast into its own header also requests splitting
several of its dependent types and their dependencies into their own
headers as well.

v2: Add standalone compilation tests

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190308132522.21573-2-chris@chris-wilson.co.uk