i965: Properly return *RESET* status in glGetGraphicsResetStatusARB

The glGetGraphicsResetStatusARB from ARB_robustness extension always
returns GUILTY_CONTEXT_RESET_ARB and never returns NO_ERROR for guilty
context with LOSE_CONTEXT_ON_RESET_ARB strategy.  This is because Mesa
returns GUILTY_CONTEXT_RESET_ARB if batch_active !=0 whereas kernel
driver never reset batch_active and this variable always > 0 for guilty
context.  The same behaviour also can be observed for batch_pending and
INNOCENT_CONTEXT_RESET_ARB.

But ARB_robustness spec says:

  If a reset status other than NO_ERROR is returned and subsequent calls
  return NO_ERROR, the context reset was encountered and completed. If a
  reset status is repeatedly returned, the context may be in the process
  of resetting.

  8. How should the application react to a reset context event?
  RESOLVED: For this extension, the application is expected to query the
  reset status until NO_ERROR is returned. If a reset is encountered, at
  least one *RESET* status will be returned. Once NO_ERROR is
  encountered, the application can safely destroy the old context and
  create a new one.

The main problem is the context may be in the process of resetting and
in this case a reset status should be repeatedly returned.  But looks
like the kernel driver returns nonzero active/pending only if the
context reset has already been encountered and completed.  For this
reason the *RESET* status cannot be repeatedly returned and should be
returned only once.

The reset_count and brw->reset_count variables can be used to control
that glGetGraphicsResetStatusARB returns *RESET* status only once for
each context.  Note the i915 triggers reset_count twice which allows to
return correct reset count immediately after active/pending have been
incremented.

v2 (idr): Trivial reformatting of comments.

Signed-off-by: Pavel Popov <pavel.e.popov@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: "10.1 10.2" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 8dc4a98c44a824630f3cc234136833dbac9a1f4c)

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