The backend begins with a series of #defines of libdrm tokens, in case
the libdrm we build against is too old.

Commit efdebbc4

 ("configure.ac: bump libdrm requirement to 2.4.68")
did what it said on the box; since we now depend on a relatively modern
libdrm, we can get rid of most of our compatibility defines.

DRM_CAP_TIMESTAMP_MONOTONIC was added in libdrm 2.4.47 (f8f1f6e37ae2).
DRM_CLIENT_CAP_UNIVERSAL_PLANES was added in libdrm 2.4.55
(8fc62ca8ac01).
DRM_CAP_CURSOR_WIDTH and HEIGHT were added in libdrm 2.4.68
(cc9a53f076d4).

Remove these four fallback definitions.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derek.foreman.samsung@gmail.com>