drm/amdgpu: Limit vm max ctx number to 4096

driver need to reserve resource for each ctx for
some hw features. so add this limitation.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>