drm/amdgpu: csa_vaddr should not larger than AMDGPU_GMC_HOLE_START

After removing unnecessary VM size calculations,
vm_manager.max_pfn would reach 0x10,0000,0000
max_pfn << AMDGPU_GPU_PAGE_SHIFT exceeding AMDGPU_GMC_HOLE_START
would cause GPU reset.

Signed-off-by: wentalou <Wentao.Lou@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>