drm/radeon: Move fbdev object helpers before struct fb_ops et al

Move the helpers for creating and destroying fbdev GEM objects
to the top of the source file. Makes them available to fb_ops
functions. This will allow to implement framebuffer cleanup in
fb_destroy. No functional changes.

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>