drm/nv50: calculate vram reordering block size

Will be used at a later point when we plug in an alternative VRAM memory
manager for GeForce 8+ boards.

Based on pscnv code to do the same.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Marcin Kościelnicki <koriakin@0x04.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>