vc4: Try to fix the HW-2116 workaround.

vc4: Try to fix the HW-2116 workaround.

We were incrementing the count at the end of vc4_start_draw(), except that
that function returns immediately if we've already started drawing on this
batch.  It also failed to count the statechanges from the GFXH-515
workaround.

This incidentally allows repeated glClear() to be coalesced, because the
fast clears aren't counted in draw_calls_queued any more.  Fixes most of
the extra flushes in Processing, which emits glClear(Z|S); glClear(Z);
glClear(C) during its frame setup.

Improves performance of Processing's QuadRendering demo at 5000 quads by
3.33538% +/- 2.05846% (n=21 before, 15 after)