drm/radeon: Implement client-based fbdev emulation

Implement fbdev emulation on top of struct drm_client and its helpers.
Replaces ad-hoc interfaces for restoring and closing fbdev emulation with
per-client callbacks for hotplugging, restoring and unregistering.

A single function, radeon_fbdev_setup(), starts fbdev emulation after
the DRM device has been registered. Hence, fbdev acts like a regular
DRM client.

The setup call prepares the fbdev emulation and invokes connector
hotplugging. The first successful hotplug event initializes fbdev
emulation with a framebuffer, device file, etc.

Unregistering depends on the hotplug status. Fully initialized emulation
is cleaned up through drm_fb_helper_unregister_info() and fb_destroy.
For prepared-only setups, unregistering unprepares the emulation and
releases all resources. In both cases, fbdev emulation will be cleaned
up.

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>