Remove the implementation of struct drm_driver.lastclose. The hook
was only necessary before in-kernel DRM clients existed, but is now
obsolete. The code in amdgpu_driver_lastclose_kms() is performed by
drm_lastclose().

v2:
- update commit message

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240812083000.337744-3-tzimmermann@suse.de