backend-drm: bump libdrm requirement to newer version (2.4.91)
Since major distributions supports this version (2.4.91), bump libdrm requirement. Here are some links to prove:
- Debian Stable: 2.4.97 - https://tracker.debian.org/pkg/libdrm
- openSUSE Leap 15.1: 2.4.97 - https://software.opensuse.org/package/libdrm
- Ubuntu Xenial: 2.4.91 - https://packages.ubuntu.com/search?keywords=libdrm-dev
- Fedora 29 - 2.4.97 - https://apps.fedoraproject.org/packages/libdrm
- Arch Linux: 2.4.100 - https://www.archlinux.org/packages/extra/x86_64/libdrm/
This should address the @evelikov comment at #250 (comment 175778)