drm/i915: Split execlists hardware status page initialisation from setup

Split the hardware status page into setup and initialisation,
where setup means setting up the driver state to support the
engine, and initialization means programming the hardware
with the before set up state.

This way the design matches the design of the engine setup/init
code which is split in the same fashion and it enables the
stages to be used in a balanced fashion (engine setup - hws
setup, engine init - hws init).

This will enable the upcoming improvements to slot in without
any kludges on the GPU reset path.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>