drm/i915: Add interface to reserve fence registers for vGPU

In the past, vGPU alloc fence registers by walking through mm.fence_list
to find fence which pin_count = 0 and vma is empty. vGPU may not find
enough fence registers this way. Because a fence can be bind to vma even
though it is not in using. We have found such failure many times these
days.

An option to resolve this issue is that we can force-remove fence from
vma in this case.

This patch added two new api to the fence management code:
 - i915_reserve_fence() will try to find a free fence from fence_list
   and force-remove vma if need.
 - i915_unreserve_fence() reclaim a reserved fence after vGPU has
   finished.

With this change, the fence management is more clear to work with vGPU.
GVTg do not need remove fence from fence_list in private.

v3: (Chris)
  - Add struct_mutex lock assertion.
  - Only count for unpinned fence.

v2: (Chris)
  - Rename the new api for symmetry.
  - Add safeguard to ensure at least 1 fence remained for host display.

Signed-off-by: Changbin Du <changbin.du@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1504512061-5892-1-git-send-email-changbin.du@intel.com


Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>