anv: Distinguish between subpass setup and subpass start

anv: Distinguish between subpass setup and subpass start

vkCmdBeginRenderPass, vkCmdNextSubpass, and vkBeginCommandBuffer with
VK_COMMAND_BUFFER_USAGE_RENDER_PASS_CONTINUE_BIT, all *setup* the
command buffer for recording commands for some subpass.  But only the
first two, vkCmdBeginRenderPass and vkCmdNextSubpass, can *start*
a subpass.

Therefore, calling anv_cmd_buffer_begin_subpass() inside
vkCmdBeginCommandBuffer is misleading. Clarify its purpose by renaming
it to anv_cmd_buffer_set_subpass() and adding comments.