drm/vkms: Convert to DRM's vblank timer

In the Linux kernel, the following vulnerability has been resolved:

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.