drm/amdgpu: increase the reserved VM size to 2MB

Ideally this should be a multiple of the VM block size.
2MB should at least fit for Vega/Navi.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>