radeon: Only destroy fbdev framebuffer if it was initialized

Fixes crash when trying to unload the radeon module before the fbdev
framebuffer was initialized, which can happen since the DRM fbdev helper
code supports deferred setup.

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