drm/amdgpu: change default of amdgpu_vram_page_split to 512

512 is enough for one PD entry on Vega10.

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>