drm/amd/display: Check stream pointer is initialized before accessing

drm/amd/display: Check stream pointer is initialized before accessing

[why & how]
We calculate static screen wait frames based
on the current timing info in the active stream.
If stream is not initialized, then we should skip
the calculation and go with the default values.

Reviewed-by: Gabe Teeger <gabe.teeger@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Sung Joon Kim <sungjoon.kim@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>