drm/amdgpu: don't init fbdev if we don't have any connectors

Don't init fbdev if we don't have connectors. E.g., if you have
a PX laptop with the displays attached to an IGP with no driver
support, you may end up with a blank screen rather than falling
back to vesa, etc.

Based on a similar radeon patch from Rob Clark.

Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_fb.c

···
 	if (!adev->mode_info.mode_config_initialized)
 		return 0;
 
+	/* don't init fbdev if there are no connectors */
+	if (list_empty(&adev->ddev->mode_config.connector_list))
+		return 0;
+
 	/* select 8 bpp console on low vram cards */
 	if (adev->mc.real_vram_size <= (32*1024*1024))
 		bpp_sel = 8;