drm/i915: Adds graphic address space ballooning logic

drm/i915: Adds graphic address space ballooning logic

With Intel GVT-g, the global graphic memory space is partitioned by
multiple vGPU instances in different VMs. The ballooning code is called
in i915_gem_setup_global_gtt(), utilizing the drm mm allocator APIs to
mark the graphic address space which are partitioned out to other vGPUs
as reserved. With ballooning, host side does not need to translate a
grahpic address from guest view to host view. By now, current implementation
only support the static ballooning, but in the future, with more cooperation
from guest driver, the same interfaces can be extended to grow/shrink the
guest graphic memory dynamically.

v2:
take Chris and Daniel's comments:
	- no guard page between different VMs
	- use drm_mm_reserve_node() to do the reservation for ballooning,
	instead of the previous drm_mm_insert_node_in_range_generic()

v3:
take Daniel's comments:
	- move ballooning functions into i915_vgpu.c
	- add kerneldoc to ballooning functions

v4:
take Tvrtko's comments:
	- more accurate comments and commit message

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Eddie Dong <eddie.dong@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>