meson: friendly error for simple-dmabuf-drm

This is probably the most annoying ones. Some distributions do not even package
a libdrm_${driver} if the driver's hardware does not occur on the CPU
architecture, e.g. Debian x86_64 does not have libdrm_etnaviv.

Helps people to disable those.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>