drm/amd/display: fix null pointer dereference

While setting cursor position in case of mpo,
input_pixel_processor is not available for underlay,
hence add check of the same to avoid null pointer
access issue.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>