drm/amd/display: Handle get crtc position error

[Why]
dc_stream_get_crtc_position can return false.
This was unhandled in delay_cursor_until_vupdate

[How]
If dc_stream_get_crtc_position returns false, something
is weird.  Don't delay.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Bhawanpreet Lakha &lt;Bhawanpreet <Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>