drm/qxl: Use vblank timer

Use a vblank timer to simulate the vblank interrupt. The DRM vblank
helpers provide an implementation on top of Linux' hrtimer. Qxl
enables and disables the timer as part of the CRTC. The atomic_flush
callback sets up the event. Like vblank interrupts, the vblank timer
fires at the rate of the display refresh.

Most userspace limits its page flip rate according to the DRM vblank
event. Qxl's virtual hardware does not provide vblank interrupts, so
DRM sends each event ASAP. With the fast access times of virtual display
memory, the event rate is much higher than the display mode's refresh
rate; creating the next page flip almost immediately. This leads to
excessive CPU overhead from even small display updates, such as moving
the mouse pointer.

This problem affects qxl and all other virtual displays. See [1] for
a discussion in the context of hypervdrm.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/dri-devel/SN6PR02MB415702B00D6D52B0EE962C98D46CA@SN6PR02MB4157.namprd02.prod.outlook.com/ # [1]
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://lore.kernel.org/r/20251008122911.231674-1-tzimmermann@suse.de

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