tests/amdgpu: fix bo eviction test issue

On Raven2/Picasso, the default VRAM size is 2048M,
and the default GTT size is 3072M. If max_allocation
of VRAM exceeds half of GTT size, GTT memory can't
hold evicted bo from VRAM and bo in itself at the
same time. Then amdgpu_cs_list_validate will failed
with "Not enough memory for command submission" error.

NOTE:
The installed DRAM should be larger than 8GB,
if the VRAM size is 2048M.

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>