drm/amdgpu: Run DRM default client setup

Call drm_client_setup() to run the kernel's default client setup
for DRM. Set fbdev_probe in struct drm_driver, so that the client
setup can start the common fbdev client.

The amdgpu driver specifies a preferred color mode depending on
the available video memory, with a default of 32. Adapt this for
the new client interface.

v5:
- select DRM_CLIENT_SELECTION
v2:
- style changes

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Xinhui Pan <Xinhui.Pan@amd.com>
Tested-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-66-tzimmermann@suse.de