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 <alexander.deucher@amd.com>