drm/i915: return the correct usable aperture size under gvt environment

I915_GEM_GET_APERTURE ioctl is used to probe aperture size from userspace.
In gvt environment, each vm only use the ballooned part of aperture, so we
should return the correct available aperture size exclude the reserved part
by balloon.

v2: add 'reserved' in struct i915_address_space to record the reserved size
in ggtt (Chris)

v3: remain aper_size as total, adjust aper_available_size exclude reserved
and pinned. UMD driver need to adjust the max allocation size according to
the available aperture size but not total size. KMD return the correct
usable aperture size any time (Chris, Joonas)

v4: decrease reserved in deballoon (Joonas)

v5: add onion teardown in balloon, add vgt_deballoon_space (Joonas)

v6: change title name (Zhenyu)

v7: code style refine (Joonas)

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Suggested-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Weinan Li <weinan.z.li@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1496198152-14175-1-git-send-email-weinan.z.li@intel.com
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>