anv: narrow flushing of the render target to buffer writes

anv: narrow flushing of the render target to buffer writes

In commit 9a7b3199037ac4 ("anv/query: flush render target before
copying results") we tracked all the render target writes to apply a
flushes in the vkCopyQueryResults(). But we can narrow this down to
only when we write a buffer (which is the only input of
vkCopyQueryResults).

v2: Drop newer render target write flags introduce by 1952fd8d2ce905
    ("anv: Implement VK_EXT_conditional_rendering for gen 7.5+")

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net> (v1)