drm/i915/gtt: Do not initialize drm_mm twice.

It would be initialized just moments later by i915_init_vm.

Rearrange the code such that i915_init_vm() is next to its callers
inside i915_gem_gtt (and so we can make it static). After removing the
dance around the files, it is clear that we are repeating some work
inside the initializers (such as calling drm_mm_init() multiple times),
so take advantage of the refactor to also remove some redundant code and
clean up the interface.

v2: Commit msg update,
    s/i915_init_vm/i915_address_space_init, move to i915_gem_gtt.c,
    init address_space during i915_gem_setup_global_gtt for ggtt.
v3: Do not init global_link - we are adding it to vm_list moments later,
    make i915_address_space_init static, use OOP style parameter order.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>