drm/radeon: handle runtime pm in fbcon (v2)

Ported from nouveau.

v2: re-enable runtime autosuspend in the error case

Acked-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>