i830: Compute initial number of vertices from remaining batch space

i830: Compute initial number of vertices from remaining batch space

In order to prevent an overflow of the batch buffer when emitting
triangles, we need to limit the initial primitive to fit within the
current batch. To do we need to measure the remaining space and thence
compute the maximum number of vertices that fit into that space.

Reported-by: Kurt Roeckx <kurt@roeckx.be>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=41495
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>
NOTE: This is a candidate for release branches.