drm/amd/display: Use NULL for pointer assignment in copy_stream_update_to_stream

Clang warns:

../drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:1965:26: warning:
expression which evaluates to zero treated as a null pointer constant of
type 'struct dc_dsc_config *' [-Wnon-literal-null-conversion]
                                update->dsc_config = false;
                                                     ^~~~~
../drivers/gpu/drm/amd/amdgpu/../display/dc/core/dc.c:1971:25: warning:
expression which evaluates to zero treated as a null pointer constant of
type 'struct dc_dsc_config *' [-Wnon-literal-null-conversion]
                        update->dsc_config = false;
                                             ^~~~~
2 warnings generated.

Fixes: f6fe4053 ("drm/amd/display: Use a temporary copy of the current state when updating DSC config")
Link: https://github.com/ClangBuiltLinux/linux/issues/777


Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>