drm/i915: Move calling engine->init_hw() to its own function

Just a simple refactor to move a loop and some support code out of
i915_gem_init_hw(). This is in preparation for avoiding a race between
the tasklet writing to ELSP whilst simultaneously being written by
engine->init_hw() following a GPU reset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170208143033.11651-1-chris@chris-wilson.co.uk