Log Message:
Reduce mimimal requirement of libdrm from 2.4.20 to 2.4.15, allow OpenBSD-5.1
builtin one, or failed to buildlink with MesaLib, because buitin MesaLib is
allowed but depending libdrm not.

* Previous pkgsrc version is 2.4.13
* NetBSD-5.1's one is 2.4.11
* NetBSD-6.0_BETA's one is 2.4.23

So actual dependency should not be changed.