drm/amdgpu: Pessimistic availability based on rounded up allocations

Separately accumulate a statistic of rounded up allocations to use
to report availability, with a view to increasing the likelihood a
buffer object can be successfully allocated at exactly the size
reported by the availability API.

Signed-off-by: Daniel Phillips <daniel.phillips@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>