drm/nouveau/debugfs: Move a variable assignment behind a null pointer check in nouveau_debugfs_pstate_get()

The address of a data structure member was determined before
a corresponding null pointer check in the implementation of
the function “nouveau_debugfs_pstate_get”.

Thus avoid the risk for undefined behaviour by moving the assignment
for the variable “ctrl” behind the null pointer check.

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/5b7b99f7-1692-74e5-4b1e-cfa14dad7c57@web.de