drm/nv40: allocate ctxprog with kmalloc

Some archs defconfigs have CONFIG_FRAME_WARN set to 1024, which lead to this
warning:
drivers/gpu/drm/nouveau/core/engine/graph/ctxnv40.c: warning: the frame size
of 1184 bytes is larger than 1024 bytes

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>