drm/amd/display: fix boot-up on vega10

Fixing null-deref on Vega10 due to regression after
'fix cursor related Pstate hang' change.
Added null checks in setting cursor position.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>