drm/radeon: Improve fbdev object-test helper

Look up the framebuffer GEM object in fbdev object test with the
respective helper drm_gem_fb_get_obj(). The look-up helper warns if
no GEM object has been installed. Upcasting types prevents runtime
type checking, so avoid upcast to struct radeon_bo.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>