media-libs/mesa: Bump libdrm dependency to 2.4.56.

Bugzilla: https://bugs.gentoo.org/show_bug.cgi?id=519384
media-libs/mesa: rebuild is required when llvm is updated

https://bugs.gentoo.org/510774 by Alexander Tsoy <alexander@tsoy.me>

Package-Manager: portage-2.2.8-r1
Use slot operator everywhere for MULTILIB_USEDEP dependency

Bugzilla: https://bugs.gentoo.org/show_bug.cgi?id=518504
Merge branch 'master' of git://git.overlays.gentoo.org/proj/x11