svga: Add an environment variable to force coherent surface memory

svga: Add an environment variable to force coherent surface memory

The vmwgfx driver supports emulated coherent surface memory as of version
2.16. Add en environtment variable to enable this functionality for
texture- and buffer maps: SVGA_FORCE_COHERENT.
This environment variable should be used for testing only.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>