drm/nouveau: Calculate reserved VRAM for PRAMIN value before use.

'drm/nouveau: rework vram init/fini ordering a little' changed
the order of instmem.init() and nouveau_mem_vram_init() which
resulted in using ramin_rsvd_vram before it was calculated and
failing to init any accel on pre-NV50 cards.

Since it's only used on <NV50 just calculate it where it's needed
and leave it as default 0 for NV50.

Signed-off-by: Younes Manton <younes.m@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>