anv: set UAV coherence required bit when needed

anv: set UAV coherence required bit when needed

The same we do in the OpenGL driver (comment copied from there).

This is required to ensure that we execute the fragment shader stage when
side-effects (such as image or ssbo stores) are present but there are no
color writes.

I found this while writing a test to check rendering to a framebuffer
without attachments where the fragment shader does not produce any
color outputs but writes to an image via imageStore(). Without this patch
the fragment shader does not execute and the image is not written,
which is not correct.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>