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>
10 jobs for !915 with fbsd-dmabuf-reenable in 15 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Containers Build
canceled #312559
debian

00:00:15

 
  Build+Test
skipped #312561
meson-clang
skipped #312565
meson-clover
skipped #312564
meson-main
skipped #312560
meson-swr-glvnd
skipped #312566
meson-vulkan
skipped #312568
scons-llvm
skipped #312567
scons-nollvm
skipped #312562
scons-swr
skipped #312563
scons-win64