wined3d: Only drop WINED3D_MAP_DISCARD right before mapping the buffer object in wined3d_buffer_map().

Dropping WINED3D_MAP_DISCARD too early means we go through the system memory
path unnecessarily. Worse, as mentioned in an earlier comment, Darkstar One
depends on the entire buffer being uploaded in case of WINED3D_MAP_DISCARD
maps, and dropping the flag too early prevents that as well. This fixes a
regression introduced by commit 15d53761.
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>