freedreno: a bit more debug

Signed-off-by: Rob Clark <robdclark@gmail.com>
freedreno: fix bugs with clear

1) Deal with colormask properly to handle clears of only depth and/or
stencil without a clear of the color buffer.
2) A clear not followed by a draw would result in the gmem->mem step
being skipped.. which isn't very useful.

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