Bump libdrm version requirement and remove unecessary ifdef's
There are some features that are available only if the user's libdrm version is not too old: format modifiers require at least libdrm 2.4.71, atomic API at least 2.4.78 and blob formats at least 2.4.83. Commit 1/2 bumps libdrm requirement to 2.4.83 (the oldest version that support these features). Major distributions already support libdrm 2.4.91, so it's safe to apply the commit. Here are some links that prove this:
- 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/
Commit 2/2 removes #ifdef checks (for HAVE_DRM_ADDFB2_MODIFIERS, HAVE_DRM_ATOMIC and HAVE_DRM_FORMATS_BLOB) that are unnecessary for libdrm versions >= 2.4.83. Also, the checks to enable these flags are removed from libweston/backend-drm/meson.build. This makes the code cleaner and solves issue #250 (closed) ("Move #ifdef HAVE_DRM_ATOMIC code into new file").