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

BugLink: http://bugs.launchpad.net/bugs/1580526

commit e5f243bd upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Alberto Milone <alberto.milone@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>