drm/i915: Move GT powersaving init to i915_gem_init()

GT powersaving is tightly coupled to the request infrastructure. To
avoid complications with the order of initialisation in the next patch
(where we want to send requests to hw during GEM init) move the
powersaving initialisation into the purview of i915_gem_init().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110142634.10551-3-chris@chris-wilson.co.uk