drm/amd/display: Pass non-null to dcn20_validate_apply_pipe_split_flags

commit 55595987 upstream.

[WHAT & HOW]
"dcn20_validate_apply_pipe_split_flags" dereferences merge, and thus it
cannot be a null pointer. Let's pass a valid pointer to avoid null
dereference.

This fixes 2 FORWARD_NULL issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[ dcn20 and dcn21 were moved from drivers/gpu/drm/amd/display/dc to
  drivers/gpu/drm/amd/display/dc/resource since commit
  8b8eed05 ("drm/amd/display: Refactor resource into component directory").
  The path is changed accordingly to apply the patch on 6.6.y. ]
Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>