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: f6fe4053b91f ("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 <[e-mail]>
Signed-off-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dc/core/dc.c

···
 			if (!dc->res_pool->funcs->validate_bandwidth(dc, dsc_validate_context, true)) {
 				stream->timing.dsc_cfg = old_dsc_cfg;
 				stream->timing.flags.DSC = old_dsc_enabled;
-				update->dsc_config = false;
+				update->dsc_config = NULL;
 			}
 
 			dc_release_state(dsc_validate_context);
 		} else {
 			DC_ERROR("Failed to allocate new validate context for DSC change\n");
-			update->dsc_config = false;
+			update->dsc_config = NULL;
 		}
 	}
 }