drm/radeon: Remove struct radeon_fbdev

Both data fields in struct radeon_fbdev, the framebuffer and the
device, are already available in struct drm_fb_helper. Simplify
radeon by converting all callers and removing struct radeon_fbdev.

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>