drm/radeon: rework fbdev handling on chips with no connectors

Move all the logic to radeon_fb.c and add checks to functions
called frome elsewhere.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=112781



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org