freedreno: skip depth resolve if not written

freedreno: skip depth resolve if not written

For multi-pass rendering, it is common to keep the same depth buffer
from previous pass, to discard geometry that would be hidden by later
draws.  In the later passes with depth-test enabled, but depth-write
disabled, there is no reason to do gmem2mem resolve.

TODO probably do something similar for stencil.. although stencil
buffer isn't used as commonly these days

Signed-off-by: Rob Clark <robdclark@gmail.com>