i965/blorp: Support overriding destination alpha to 1.0.

Currently, Blorp requires the source and destination formats to be
equal.  However, we'd really like to be able to blit between XRGB and
ARGB formats; our BLT engine paths have supported this for a long time.

For ARGB -> XRGB, nothing needs to occur: the missing alpha is already
interpreted as 1.0.  For XRGB -> ARGB, we need to smash the alpha
channel to 1.0 when writing the destination colors.  This is fairly
straightforward with blending.

For now, this code is never used, as the source and destination formats
still must be equal.  The next patch will relax that restriction.

NOTE: This is a candidate for the 9.1 branch.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>
Tested-by: Martin Steigerwald <[email protected]>

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