drm/nouveau/bios/power_budget: Move an expression into a macro call parameter in nvbios_power_budget_header()

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

Thus avoid the risk for undefined behaviour by moving the usage
of an expression into a parameter for a macro call in one if branch.

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/529fe82e-59f0-71a2-1e33-b051f833b67c@web.de