gallium/svga: Upload only parts of user-buffers that we actually use

gallium/svga: Upload only parts of user-buffers that we actually use

Stream user buffer contents rather than trying to maintain persistent
host / hardware copies.
Resulting negative array offsets are not allowed by the hardware,
(well, at least not according to header files), so adjust index bias
to make all array offsets positive.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>