Bump libdrm dependency to 2.4.95
96bef051 "drm-backend: add support for writeback connectors" started using DRM_MODE_CONNECTOR_WRITEBACK and DRM_CLIENT_CAP_WRITEBACK_CONNECTORS. These were introduced in libdrm 2.4.95.
According to !311 (merged) Ubunut Xenial is the only mentioned distribution that does not provide a libdrm new enough. I think that is fine to drop now, 2016 was a good while ago.
Libdrm 2.4.95 also introduced DRM_CLIENT_CAP_ASPECT_RATIO, DRM_MODE_PICTURE_ASPECT_64_27, DRM_MODE_PICTURE_ASPECT_256_135.
The fallback definitions for the above are dropped.