drm/amd/display: Remove unnecessary NULL check in commit_planes_for_stream()

Smatch complains that:

    drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:3369 commit_planes_for_stream()
    warn: variable dereferenced before check 'stream' (see line 3114)

The 'stream' pointer cannot be NULL and the check can be removed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>