drm/i915: Move stolen memory initialization earlier during loading

The only device specific dependency of the stolen memory setup is the
MMIO mapping and the stolen memory size. Both are already available in
i915_gtt_init(), so move the stolen initialization to there. The
clean-up code for i915_gtt_init() is in i915_global_gtt_cleanup(), so
move the stolen memory clean-up code there too.

This will be needed by an upcoming patch that needs the details of the
memory we reserve, but the change is also part of our generic goal to
move the initialization of resources with no or little dependencies on
other device specific resources towards the beginning of the init
sequence.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453209992-25995-8-git-send-email-imre.deak@intel.com