winsys/svga: Optionally avoid caching buffer maps

winsys/svga: Optionally avoid caching buffer maps

Mapping of graphics kernel buffers is quite costly. Therefore the svga
drm winsys caches all kernel buffer maps. However, that may lead to
less testing coverage of the unmap paths and (possibly) processes running
out of virtual memory space. Introduce a possibility to avoid that caching
by setting the environment variable SVGA_FORCE_KERNEL_UNMAPS to 1.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Matthew McClure <mcclurem@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4804>