The critical bug that was patched from libdrm 2.4.49 would lead to crashes when running some OpenGL applications or even Google Chrome from the open-source AMD RadeonSI driver. The bug documenting the issue is on the FreeDesktop.org BugZilla.
There was a fix committed for this issue, "radeon: Update unaligned offset for 2D->1D tiling transition on SI." The only other change squeezing into libdrm 2.4.50 is an Intel change by Keith Packard for tracking known DRI PRIME buffers for re-use that were previously already passed to the game/application.
The libdrm 2.4.50 release announcement can be found on dri-devel.