nouveau: disallow pushbuf BOs in multiple memory types

Under certain circumstances it's possible for libdrm to decide to move
a GART|VRAM pushbuf to be VRAM-only.  This causes the kernel to reject
the command submission on GF8 and up, due to a stricter policy where
buffers are only allowed to move to memory types that were specified
at creation time.

The simplest fix for this is to force the creation-time memory type for
the lifetime of the push buffer.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>