drm/radeon: restore the fbdev mode in lastclose

restore the fbdev state if a drm app like X is killed.

Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>