Libdrm provides headers that are useful even without libdrm.so itself,
particularly drm_fourcc.h. Therefore promote libdrm as a hard build-time
dependency of libweston core so that we can always rely on libdrm
headers.

This does not affect any runtime dependencies. Specifically, no runtime
dependency to libdrm.so is added in any build configuration.

Currently only gl-renderer is using drm_fourcc.h. Now we can drop the
GL_RENDERER check from configure.ac and just use LIBDRM_CFLAGS.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
[Pekka, from Quentin: just drop have_libdrm var completely]
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>