drm/nouveau: Removes unnecessary args check in nouveau_uvmm_sm_prepare

drm/nouveau: Removes unnecessary args check in nouveau_uvmm_sm_prepare

Checking `args` after calling `op_map_prepare` is unnecessary since
if `op_map_prepare` was to be called with  NULL args, it would lead
to a NULL pointer dereference, thus never hitting that check.

Hence remove the check and add a note to remind users of this function
to ensure that args != NULL when calling this function for a map
operation as it was suggested by Danilo [1].

[1] https://lore.kernel.org/lkml/6a1ebcef-bade-45a0-9bd9-c05f0226eb88@redhat.com

Suggested-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Yuran Pereira <yuran.pereira@hotmail.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/GV1PR10MB65637F4BAABFE2D8E261E1DCE8B0A@GV1PR10MB6563.EURPRD10.PROD.OUTLOOK.COM