anv: Make vkBeginCommandBuffer reset the command buffer

anv: Make vkBeginCommandBuffer reset the command buffer

If its the command buffer's first call to vkBeginCommandBuffer, we must
*initialize* the command buffer's state. Otherwise, we must *reset* its
state. In both cases, let's use anv_ResetCommandBuffer.

From the Vulkan 1.0 spec:

   If a command buffer is in the executable state and the command buffer
   was allocated from a command pool with the
   VK_COMMAND_POOL_CREATE_RESET_COMMAND_BUFFER_BIT flag set, then
   vkBeginCommandBuffer implicitly resets the command buffer, behaving
   as if vkResetCommandBuffer had been called with
   VK_COMMAND_BUFFER_RESET_RELEASE_RESOURCES_BIT not set. It then puts
   the command buffer in the recording state.