freedreno: document debug flag

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Brian Paul <brianp@vmware.com>

src/gallium/docs/source/debugging.rst

@@ -81,6 +81,10 @@ Debug :ref:`flags` for the llvmpipe driver.
 
 Number of threads that the llvmpipe driver should use.
 
+.. envvar:: FD_MESA_DEBUG <flags> (0x0)
+
+Debug :ref:`flags` for the freedreno driver.
+
 
 .. _flags: