drm/virtio: Spelling fixes

While making a spelling mistake myself for `git grep kvalloc`
I found that the only file has such a typo. Fix it and update
to the standard de facto of how we refer to the functions.
Also spell usr-out as user-out, it seems this driver uses its
own terminology nobody else can decypher, make it more readable.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231219151955.2477488-1-andriy.shevchenko@linux.intel.com