Fix build when HAVE_LIBDRM isn't defined

Fix build when HAVE_LIBDRM isn't defined

make[4]: Entering directory '/wip/mesa/build/src/gallium/targets/dri'
  CXXLD    gallium_dri.la
../../../../src/gallium/auxiliary/pipe-loader/.libs/libpipe_loader_static.a(libpipe_loader_static_la-pipe_loader.o): In function `pipe_loader_get_driinfo_xml':
/mesa/build/src/gallium/auxiliary/pipe-loader/../../../../../src/gallium/auxiliary/pipe-loader/pipe_loader.c:117: undefined reference to `pipe_loader_drm_get_driinfo_xml'

b4ff5e90 uses pipe_loader_get_driinfo_xml() unconditionally in
pipe_loader.c, but it's definition in pipe_loader_get_driinfo_xml() is only
built if HAVE_LIBDRM.

Arrange to always use the default XML if HAVE_LIBDRM isn't defined.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>