vc4: Start converting the driver to use vertex shaders.

vc4: Start converting the driver to use vertex shaders.

Note: This is the cutoff point where I switched from developing primarily
on the Pi to developing o the simulator.  As a result, from this point on
the code is untested on the Pi (the kernel code I have currently wasn't
rendering anything at this commit, though the simulator renders
successfully, suggesting kernel bugs).