drm/amd/display: Fix NULL ptr deref for commit_planes_to_stream

drm/amd/display: Fix NULL ptr deref for commit_planes_to_stream

[Why]
With scaling, underscan and abm changes we can end up calling
commit_planes_to_stream in commit_tail. This call uses dm_state->context
which can be NULL if the commit was a fast update.

[How]
Use dc_state instead since that can't be NULL unless the system ran
out of memory.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108912
Fixes: e64abff2f133 ("drm/amd/display: Use private obj helpers for dm_atomic_state")

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