gallium: enable dmabuf on BSD as well

The DRM_CONF_SHARE_FD code did not check for Linux, so the commit that
introduced PIPE_CAP_DMABUF broke Wayland-EGL clients on FreeBSD.

Fixes: 8ae50e60 (gallium: replace DRM_CONF_SHARE_FD with PIPE_CAP_DMABUF)
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 506ebf55)