i965: Cache register write capability checks.

Our ability to perform register writes depends on the hardware and
kernel version.  It shouldn't ever change on a per-context basis,
so we only need to check once.

Checking introduces a synchronization point between the CPU and GPU:
even though we submit very few GPU commands, the GPU might be busy doing
other work, which could cause us to stall for a while.

On an idle i7 4750HQ, this improves performance in OglDrvCtx (a context
creation microbenchmark) by 6.14748% +/- 1.6837% (n=20).  With Unigine
Valley running in the background (to keep the GPU busy), it improves
performance in OglDrvCtx by 2290.92% +/- 29.5274% (n=5).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Ben Widawsky <[email protected]>

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