anv/cmd_buffer: Decide whether or not to HiZ clear up-front

anv/cmd_buffer: Decide whether or not to HiZ clear up-front

This moves the decision out of begin_subpass and into BeginRenderPass
like the decision for color clears.  We use a similar name for the
function for depth/stencil as for color even though no aux usage is
really getting computed.

v2 (Jason Ekstrand):
 - Don't always disable HiZ clears by accident
 - Use the initial layout to decide whether to do fast clears

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>