vl: Enable DRM by default.

vl: Enable DRM by default.

If libdrm is found the pipe loader enables drm anyway, and that is
pretty much the only extra dependency this code has.

This enables creating libva display using a drm fd without having
to enable the DRM (GBM really) backend of EGL, which is completely
unrelated.

Leaving the X11 platforms alone as they would still result in the
additional inclusion of extra deps.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>