drm/i915/execlists: Refactor common engine setup

Move all of the constant assignments up front and into a common
function. This is primarily to ensure the backpointers are set as early
as possible for later use during initialisation.

v2: Use a constant struct so that all the similar values are set
together.
v3: Sanitize the engine's IMR to disable any potential interrupt before
we are ready (enabled in init_hw).
v4: Ignore the engine's IMR, to be resolved later

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462545621-30125-2-git-send-email-chris@chris-wilson.co.uk