drm/amdgpu: Remove rounding from vram allocation path

Rounding up allocations in the allocation path caused test regressions,
so now just round in the availability path.

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>