i965: Query whether we have kernel support for the TIMESTAMP register once

i965: Query whether we have kernel support for the TIMESTAMP register once

Move the query for the TIMESTAMP register from context init to the
screen, so that it is only queried once for all contexts.

On 32bit systems, some old kernels trigger a hw bug resulting in the
TIMESTAMP register being shifted and the low 32bits always zero. Detect
this by repeating the read a few times and check the register is
incrementing every 80ns as expected and not stuck on zero (as would be
the case with the buggy kernel/hw.).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>