i965: Stop lying about cpp and height of a stencil buffer.

When using a separate stencil buffer, i965 requires that the pitch of
the buffer (in the 3DSTATE_STENCIL_BUFFER command) be specified as 2x
the actual pitch.

Previously this was accomplished by doubling the "cpp" and "pitch"
values stored in the intel_region data structure, and halving the
height.  However, this was confusing, and it led to a subtle (but
benign) bug: since a stencil buffer is W-tiled, its true height must
be aligned to a multiple of 64; we were accidentally aligning its faux
height to a multiple of 64, causing memory to be wasted.

Note that for window system stencil buffers, the DDX also doubles the
cpp and pitch values.  To facilitate fixing this DDX server bug in the
future, we fix the cpp and pitch values we receive from the X server
only if cpp has the "incorrect" value of 2.

Acked-by: Kenneth Graunke <[email protected]>
Reviewed-by: Chad Versace <[email protected]>

v2: Clarify comments about the DDX.

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