drm/virtio: Refactor and optimize job submission code path

Move virtio_gpu_execbuffer_ioctl() into separate virtgpu_submit.c file,
refactoring and optimizing the code along the way to ease addition of new
features to the ioctl.

The optimization is done by using optimal ordering of the job's submission
steps, reducing code path from the start of the ioctl to the point of
pushing job to virtio queue. Job's initialization is now performed before
in-fence is awaited and out-fence setup is made after sending out job to
virtio.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230416115237.798604-2-dmitry.osipenko@collabora.com