drm/amdgpu: reserve at least 4MB of VRAM for page tables v2

This hopefully helps reduce the contention for page tables.

v2: adjust maximum reported VRAM size as well

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>