drm/amdgpu: increase reserved VRAM size to 8MB

4MB reserved VRAM size which used for page tables was not enough for
some condition, increase it to 8MB to reduce page table contention.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>