drm/nouveau/gsp: Use kzalloc_flex() for r535 display funcs

drm/nouveau/gsp: Use kzalloc_flex() for r535 display funcs

struct nvkm_disp_func ends with the user flexible array member. Allocate
the r535 display function table with kzalloc_flex() instead of open-coding
the size calculation with sizeof().

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
[dropped nothing-burger sentence from commit message]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patch.msgid.link/20260508052056.1744665-1-rosenp@gmail.com