drm/ttm: Don't evict BOs outside of the requested placement range

The radeon driver uses placement range restrictions for several reasons,
in particular to make sure BOs in VRAM can be accessed by the CPU, e.g.
during a page fault.

Without this change, TTM could evict other BOs while trying to satisfy
the requested placement, even if the evicted BOs were outside of the
requested placement range. Doing so didn't free up any space in the
requested placement range, so the (potentially high) eviction cost was
incurred for no benefit.

Nominating for stable because radeon driver changes in 3.17 made this
much more noticeable than before.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84662


Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>