meson: add override_dependency when possible

This allows consumers of libdrm as a subproject to use the simpler
`dependency('libdrm', fallback : 'libdrm')` syntax, as the libdrm build
files already tell meson that they override a dependency called
"libdrm".

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