drm/nouveau/therm: Move an assignment statement behind a null pointer check in two functions

The address of a data structure member was determined before
a corresponding null pointer check in the implementation of
the functions “nvkm_fanpwm_create” and “nvkm_fantog_create”.

Thus avoid the risk for undefined behaviour by moving the assignment
for the data structure member “fan” behind two null pointer checks.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/3d0215dc-74d4-2c42-2eee-7a5fcf62b9f1@web.de