drm/amd/display: Fix driver load crash in amdgpu_dm

[Why]
This fixes an regression introduced by:
    drm/amd/display: add stream ID and otg instance in dc_stream_state

During driver initialization, a null pointer deref is raised. This is
caused by searching for a stream status in the dc->current_state before
the dc_state swap happens at the end of dc_commit_state_no_check().
Since the swap has not happened, the dc_state to be swapped in should
be searched, and not dc->current_state.

[How]
Add a function that searches for the stream status within the given
dc_state, instead of dc->current_state. Use that before the state swap
happens in dc_commit_state_no_check().

Also remove duplicate occurrences of this function in amdgpu_dm.c.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>