i965: Whack UAV bit when FS discards and there are no color writes.

i965: Whack UAV bit when FS discards and there are no color writes.

dEQP-GLES31.functional.fbo.no_attachments.* draws a quad with no
framebuffer attachments, using a shader that discards based on
gl_FragCoord.  It uses occlusion queries to inspect whether pixels
are rendered or not.

Unfortunately, the hardware is not dispatching any pixel shaders,
so discards never happen, and the full quad of pixels increments
PS_DEPTH_COUNT, making the occlusion query results bogus.

To understand why, we have to delve into the WM_INT internal
signalling mechanism's formulas.

The "WM_INT::Pixel Shader Kill Pixel" signal is defined as:

    3DSTATE_WM::ForceKillPixel == ON ||
    (3DSTATE_WM::ForceKillPixel != Off &&
     !WM_INT::WM_HZ_OP &&
     3DSTATE_WM::EDSC_Mode != PREPS &&
     (WM_INT::Depth Write Enable || WM_INT::Stencil Write Enable) &&
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     (3DSTATE_PS_EXTRA::PixelShaderKillsPixels ||
      3DSTATE_PS_EXTRA:: oMask Present to RenderTarget ||
      3DSTATE_PS_BLEND::AlphaToCoverageEnable ||
      3DSTATE_PS_BLEND::AlphaTestEnable ||
      3DSTATE_WM_CHROMAKEY::ChromaKeyKillEnable))

Because there is no depth or stencil buffer, writes to those buffers
are disabled.  So the highlighted condition is false, making the whole
"Kill Pixel" condition false.  This then feeds into the following
"WM_INT::ThreadDispatchEnable" condition:

    3DSTATE_WM::ForceThreadDispatch != OFF &&
    !WM_INT::WM_HZ_OP &&
    3DSTATE_PS_EXTRA::PixelShaderValid &&
    (3DSTATE_PS_EXTRA::PixelShaderHasUAV ||
     WM_INT::Pixel Shader Kill Pixel ||
     WM_INT::RTIndependentRasterizationEnable ||
     (!3DSTATE_PS_EXTRA::PixelShaderDoesNotWriteRT &&
      3DSTATE_PS_BLEND::HasWriteableRT) ||
     (WM_INT::Pixel Shader Computed Depth Mode != PSCDEPTH_OFF &&
      (WM_INT::Depth Test Enable || WM_INT::Depth Write Enable)) ||
     (3DSTATE_PS_EXTRA::Computed Stencil && WM_INT::Stencil Test Enable) ||
     (3DSTATE_WM::EDSC_Mode == 1 && (WM_INT::Depth Test Enable ||
                                     WM_INT::Depth Write Enable ||
                                     WM_INT::Stencil Test Enable)))

Given that there's no depth/stencil testing, no writeable render target,
and the hardware thinks kill pixel doesn't happen, all of these
conditions are false.  We have to whack some bit to make PS invocations
happen.  There are many options.

Curro suggested using the UAV bit.  There's some precedence in doing
that - we set it for fragment shaders that do SSBO/image/atomic writes
when no color buffer writes are enabled.  We can simply include discard
here too.

Fixes 64 dEQP-GLES31.functional.fbo.no_attachments.* tests.

v2: Add a comment suggested and written by Jason Ekstrand.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>