drm/vmwgfx: Move vblank handling into separate helper

drm/vmwgfx: Move vblank handling into separate helper

Decouple vblank handling from the underlying hrtimer. This will be
helpful for replacing vmwgfx's vblank timer with DRM's common
implementation.

The new helper vmw_vkms_handle_vblank_timeout() can later be used as
callback for DRM's handle_vblank call as-is. The helper also keeps the
current semantics for restarting the timer. It returns true to restart
the next vblank timeout even if it could not acquire vmwgfx's vblank
lock.

The remaining code in vmw_vkms_vblank_simulate() will be replaced by
the DRM implementation in a later patch.

v2:
- clarify return-value semantics in commit message (Zack)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patch.msgid.link/20260427150250.699768-3-tzimmermann@suse.de