i965/blorp: do not use unnecessary hw-blending support

This is really not needed as blorp blit programs already sample
XRGB normally and get alpha channel set to 1.0 automatically by
the sampler engine. This is simply copied directly to the payload
of the render target write message and hence there is no need for
any additional blending support from the pixel processing pipeline.

The blending formula is anyway broken for color components, it
multiplies the color component with itself (blend factor is the
component itself).
Alpha blending in turn would not fix the alpha to one independent
of the source but simply used the source alpha as is instead
(1.0 * src_alpha + 0.0 * dst_alpha).

Quoting Eric:

 "If we want to actually make the no-alpha-bits-present thing work,
  we need to override the bits in the surface state or in the
  generated code.  In the normal draw path, it's done for sampling
  by the swizzling code in brw_wm_surface_state.c, and the blending
  overrides is just to fix up the alpha blending stage which
  doesn't pay attention to that for the destination surface."

If one modifies piglit test gl-3.2-layered-rendering-blit to use
color component values other than zero or one, this change will
kick in on IVB. No regressions on IVB.

This is effectively revert of c0554141a9b831b4e614747104dcbbe0fe489b9d:

    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.

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

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