ci: Disable egl_ext_device_drm tests in piglit.

ci: Disable egl_ext_device_drm tests in piglit.

If the runner has a HW device that would be supported, even without
/dev/dri forwarded into the container, it will be enumerated and the tests
on llvmpipe fail with (for example):

libEGL warning: Not allowed to force software rendering when API explicitly selects a hardware device.
libEGL warning: MESA-LOADER: failed to open i965 (search paths /builds/anholt/mesa/install/lib/dri)

Given that we can't necessarily control the DRI devices present on the
runners (particularly for developers bringing their own runners to reduce
the demands on fd.o's shared resources), just skip these tests in CI.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>