i965: Try to avoid stalls on the GPU when doing glBufferSubData().

i965: Try to avoid stalls on the GPU when doing glBufferSubData().

On DOTA2, framerate on dota2-de1.dem in windowed mode on my laptop
improves by 7.69854% +/- 0.909163% (n=3).  In a microbenchmark hitting
this code path (wall time of piglit vbo-subdata-many), runtime decreases
from 0.8 to 0.05 seconds.

v2: Use out of range start/end instead of separate bool for the active
    flag (suggestion by Jordan), fix double-upload in the stalling path.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>