drm/vkms: Convert to DRM's vblank timer

drm/vkms: Convert to DRM's vblank timer

Replace vkms' vblank timer with the DRM implementation. The DRM
code is identical in concept, but differs in implementation.

Vblank timers are covered in vblank helpers and initializer macros,
so remove the corresponding hrtimer in struct vkms_output. The
vblank timer calls vkms' custom timeout code via handle_vblank_timeout
in struct drm_crtc_helper_funcs.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20250916083816.30275-4-tzimmermann@suse.de