gallium/radeon: remove buffer_unmap calls that can potentially decrease perf

gallium/radeon: remove buffer_unmap calls that can potentially decrease perf

buffer_unmap is currently a no-op on radeon and done correctly on amdgpu.
I plan to fix it for radeon, but before that, all occurences of buffer_unmap
that can negatively affect performance in the future must be removed.

There are 2 reasons for removing buffer_unmap calls:
- There is a likelihood that buffer_map will be called again, so we don't
  want to unmap yet.
- The buffer is being released, which automatically unmaps it.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>