broadcom/vc4: Fix handling of GFXH-515 workaround with a start vertex count.

We failed to take the start into account for how many vertices to draw in
this round, so we would end up decrementing count below 0, which as an
unsigned number meant we would loop until the CLs soon ran out of space.

When I wrote the code I was thinking about how to use the previously
emitted shader state (no index bias baked into the elements) by emitting
up to 65535 and then only re-emitting with bias for the second wround, but
that doesn't work if the start is over 65535.  Instead, just delay
emitting shader state until we get into the drawarrays GFXH-515 loop and
always bake the bias in when we're doing the workaround.

(cherry picked from commit 84ab48c1)