drm/nouveau/clk: Move a variable assignment behind a null pointer check in nvkm_pstate_new()

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

Thus avoid the risk for undefined behaviour by moving the assignment
for the variable “cstate” 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/6435d829-2c9b-a3f7-c8cd-cf923508a092@web.de