drm/radeon: only init fbdev if we have connectors

This fixes an issue that was noticed on an optimus/prime laptop with
a kernel that was old enough to not support the integrated intel gfx
(which was driving all the outputs), but did have support for the
discrete radeon gpu.  The end result was not falling back to VESA and
leaving the user with a black screen.

(Plus it is kind of silly to create an framebuffer device if there
are no outputs hooked up to the gpu.)

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>