freedreno: shadow textures if possible to avoid stall/flush

freedreno: shadow textures if possible to avoid stall/flush

To make batch re-ordering useful, we need to be able to create shadow
resources to avoid a flush/stall in transfer_map().  For example,
uploading new texture contents or updating a UBO mid-batch.  In these
cases, we want to clone the buffer, and update the new buffer, leaving
the old buffer (whose reference is held by cmdstream) as a shadow.

This is done by blitting the remaining other levels (and whatever part
of current level that is not discarded) from the old/shadow buffer to
the new one.

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