gallium/docs: add some freedreno compiler docs

gallium/docs: add some freedreno compiler docs

Enable the 'sphinx.ext.graphviz' extension, and add in a section for
driver specific docs, with freedreno compiler docs beneath.  The
goal is for more complete compiler docs, and hopefully some docs about
other parts of the driver (such as how tiling works, etc).

Note that there is also a Distribution -> Drivers section.  Although
that appears to be simply just a list of drivers.  Not sure if that
should move under the 'Drivers' section or left alone.  I did add a
one-line section for freedreno in the existing Distribution -> Drivers
section.

Signed-off-by: Rob Clark <robclark@freedesktop.org>