drm/i915: Split the stolen handling for GEM out of i915_dma.c

We slightly modify the initialisation sequence to move the
initialisation of the memory managers earlier and in particular before
probing outputs and detecting any existing output configuration. This is
essential if we wish to track preallocated objects and preserve them
whilst initialising GEM.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>