nv50,nvc0: make sure there's pushbuf space and that we ref the bo early

nv50,nvc0: make sure there's pushbuf space and that we ref the bo early

First off, we can't flush in the middle of a command. Secondly
requesting the extra push space might cause a flush to happen. If that
flush happens, we'd have to do the PUSH_REFN again. So instead do
PUSH_REFN after the push space request. This helps avoid rare crashes
with supertuxkart in libdrm due to assertion failures.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>