For clients that submit large batch buffers the command parser has
a substantial impact on performance. On my HSW ULT system performance
drops as much as ~20% on some tests. Most of the time is spent in the
command lookup code. Converting that from the current naive search to
a hash table lookup reduces the performance drop to ~10%.

The choice of value for I915_CMD_HASH_ORDER allows all commands
currently used in the parser tables to hash to their own bucket (except
for one collision on the render ring). The tradeoff is that it wastes
memory. Because the opcodes for the commands in the tables are not
particularly well distributed, reducing the order still leaves many
buckets empty. The increased collisions don't seem to have a huge
impact on the performance gain, but for now anyhow, the parser trades
memory for performance.

NB: Ville noticed that the error paths through the ring init code
will leak memory. I've not addressed that here. We can do a follow
up pass to handle all of the leaks.

v2: improved comment describing selection of hash key mask (Damien)
replace a BUG_ON() with an error return (Tvrtko, Ville)
commit message improvements
Signed-off-by: Brad Volkin <bradley.d.volkin@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>