i965: Implement Pause/ResumeTransformfeedback driver hooks on Gen7+.

The ARB_transform_feedback2 extension introduces the ability to pause
and resume transform feedback sessions.  Although only one can be active
at a time, it's possible to switch between multiple transform feedback
objects while paused.

In order to facilitate this, we need to save/restore the SO_WRITE_OFFSET
registers so that after resuming, the GPU continues writing where it
left off.

This functionality also exists in ES 3.0, but somehow we completely
forgot to implement it.

v2: Reduce alignment from 4096 to 64 (it seemed excessive).
v3: Use I915_GEM_DOMAIN_INSTRUCTION instead of RENDER, for consistency
    with other writes.  It shouldn't matter on IVB+.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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