drm/amd/display: Correct timing_adjust_pending flag setting.

[Why&How]
stream->adjust will be overwritten by update->crtc_timing_adjust.
We should set update->crtc_timing_adjust->timing_adjust_pending
and then overwrite stream->adjust.
Reset update->crtc_timing_adjust->timing_adjust_pending after
the assignment.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Zhongwei Zhang <Zhongwei.Zhang@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>