gallium/winsys/kms: don't unmap what wasn't mapped

gallium/winsys/kms: don't unmap what wasn't mapped

At the moment, depending on pipe transfer flags, the dumb
buffer map address can end up at either kms_sw_dt->ro_mapped
or kms_sw_dt->mapped.

When it's time to unmap the dumb buffer, both locations get unmapped,
even though one is probably initialized to 0.

That leads to the code segment getting unmapped at runtime and
crashes when trying to call into unrelated code.

This commit addresses the problem by using MAP_FAILED instead of
NULL for ro_mapped and mapped when the dumb buffer is unmapped,
and only unmapping mapped addresses at unmap time.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107098
Signed-off-by: Ray Strode <rstrode@redhat.com>
Fixes: d891f28df9a ("gallium/winsys/kms: Fix possible leak in map/unmap.")
Cc: Lepton Wu <lepton@chromium.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>