ANDROID: drm/ttm: export ttm_tt_unpopulate()

Xclipse GPU driver depends on TTM for graphics buffer allocation and
management. It is required by customers to add graphics memory swap
to improve overall memory efficiency. However TTM's swap feature can't
be used since it selects victim buffer by LRU and we can't choose a
specific buffer to swap.
Xclipse GPU driver implements its own swap feature by means of APIs of
TTM. But the problem is TTM's buffer allocations statistics in ttm_tt.c
which are local to that file. Whenever a graphic buffer is swapped out,
the size of total page allocation should be decreased but it is not
possible from the outside of ttm_tt.c.
If the statistics is not maintained well, TTM ends up swapping out TTM
buffers globally which is unexpected.

Bug: 291101811
Bug: 293812967
Change-Id: I143c705834bcc196432c3ef59b49c9ec31f2e971
Signed-off-by: Kyongho Cho <pullip.cho@samsung.com>
(cherry picked from commit 7edb035c)