drm/virtio: Fix type of dma-fence context variable

Type of DMA fence context is u64. Fence-waiting code uses u32 for the
context variable, fix it.

Fixes: e4812ab8 ("drm/virtio: Refactor and optimize job submission code path")
Cc: <stable@vger.kernel.org> # v6.4+
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240714205009.3408298-1-dmitry.osipenko@collabora.com