[bug report] drm/amd/display: Fix ineffective setting of max bpc property

[Dan Carpenter]

Hello Stylon Wang,

Commit fa7041d9d2fc ("drm/amd/display: Fix ineffective setting of max
bpc property") from Jun 12, 2020 (linux-next), leads to the following
Smatch static checker warning:

	drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:7201 create_validate_stream_for_sink()
	error: we previously assumed 'dm_state' could be null (see line 7194)

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c
    7185 struct dc_stream_state *
    7186 create_validate_stream_for_sink(struct amdgpu_dm_connector *aconnector,
    7187                                 const struct drm_display_mode *drm_mode,
    7188                                 const struct dm_connector_state *dm_state,
    7189                                 const struct dc_stream_state *old_stream)
    7190 {
    7191         struct drm_connector *connector = &aconnector->base;
    7192         struct amdgpu_device *adev = drm_to_adev(connector->dev);
    7193         struct dc_stream_state *stream;
    7194         const struct drm_connector_state *drm_state = dm_state ? &dm_state->base : NULL;
                                                               ^^^^^^^^
                                     ^^^^^^^^^
This used check connector->state but then we changed it to dm_state instead

    7195         int requested_bpc = drm_state ? drm_state->max_requested_bpc : 8;
    7196         enum dc_status dc_result = DC_OK;
    7197 
    7198         do {
    7199                 stream = create_stream_for_sink(connector, drm_mode,
    7200                                                 dm_state, old_stream,
                                                         ^^^^^^^^

But dm_state is dereferenced on the next line without checking.  (Presumably
the NULL check can be removed).

--> 7201                                                 requested_bpc);
    7202                 if (stream == NULL) {
    7203                         DRM_ERROR("Failed to create stream for sink!\n");
    7204                         break;
    7205                 }
    7206 
    7207                 if (aconnector->base.connector_type == DRM_MODE_CONNECTOR_WRITEBACK)

regards,
dan carpenter