drm/via: use monotonic time for VIA_WAIT_IRQ

The normal DRM vblank interrupt events started using monotonic times in
commit c61eef72 ("drm: add support for monotonic vblank timestamps"),
which is useful for a number of reasons, including the possible y2038
overflow.

The VIA_WAIT_IRQ ioctl uses the same drm_wait_vblank_reply as
DRM_IOCTL_WAIT_VBLANK, but still uses wall-clock time.

This converts it to using monotonic time as well, which is more
consistent, and avoids problems with the y2038 overflow as well
as synchronization issues when the real time skips.

I could not find the matching user space that calls the VIA_WAIT_IRQ
ioctl to verify that this is safe, but it very likely is. Please
either test or review the user space side before applying this.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171127111728.1710742-1-arnd@arndb.de