drm/amd/display: Guard against null stream dereference in do flip

[Why]

During suspend under some hardware configurations can result in a
series of atomic commits with a NULL stream status - which
causes a NULL pointer dereference. This should be guarded.

[How]

Exit early from the function - if we can't access the stream then
there isn't anything that can be done here.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>