drm/nouveau/bios: Rename prom_init() and friends functions

While working at fixing powerpc headers, I ended up with the
following error.

	drivers/gpu/drm/nouveau/nvkm/subdev/bios/shadowrom.c:48:1: error: conflicting types for 'prom_init'; have 'void *(struct nvkm_bios *, const char *)'
	make[5]: *** [scripts/Makefile.build:288: drivers/gpu/drm/nouveau/nvkm/subdev/bios/shadowrom.o] Error 1

powerpc and a few other architectures have a prom_init() global function.
One day or another it will conflict with the one in shadowrom.c

Those being static, they can easily be renamed. Do it.

While at it, also rename the ops structure as 'nvbios_prom' instead of
'nvbios_rom' in order to make it clear that it refers to the
NV_PROM device.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7e0612b61511ec8030e3b2dcbfaa7751781c8b91.1647684507.git.christophe.leroy@csgroup.eu