meson: use the modern interface for pkg.generate

This produces no differences in the generated output. I've had to
manually add `requires : 'libdrm'` to libdrm_intel, otherwise libdrm
ends up in `Requires.private` instead of `Requires`.

Signed-off-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Simon Ser <contact@emersion.fr>