svga: Treat forced coherent maps as maps of persistent memory

svga: Treat forced coherent maps as maps of persistent memory

A previous commit made sure we sent a BindGBSurface command at map time
rather than at unmap time for persistent memory. To be consistent, do the
same for forced coherent maps. This makes it possible to avoid the
explicit UpdateGBSurface at unmap time for discard maps and to instead rely
on the kernel's dirty-tracking mechanism at the cost of an additional flush.

Tested with SVGA_FORCE_COHERENT=1, piglit run quick. No regressions.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4399>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4399>