anv/cmd_buffer: Move the color portion of clear_subpass into begin_subpass

anv/cmd_buffer: Move the color portion of clear_subpass into begin_subpass

This doesn't really change much now but it will give us more/better
control over clears in the future.  The one interesting functional
change here is that we are now re-emitting 3DSTATE_DEPTH_BUFFERS and
friends for each clear.  However, this only happens at begin_subpass
time so it shouldn't be substantially more expensive.

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