drm/vmwgfx: Remove unnecessary NULL checks before kvfree()

Since kvfree() already checks if its argument is NULL, an additional
check before calling kvfree() is unnecessary and can be removed.

Remove both and the following Coccinelle/coccicheck warnings reported by
ifnullfree.cocci:

  WARNING: NULL check before some freeing functions is not needed
  WARNING: NULL check before some freeing functions is not needed

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241007115131.1811-3-thorsten.blum@linux.dev