vk: Cache each render pass's number of clear ops

vk: Cache each render pass's number of clear ops

During vkCreateRenderPass, count the number of clear ops and store them
in new members of anv_render_pass:

    uint32_t num_color_clear_attachments
    bool has_depth_clear_attachment
    bool has_stencil_clear_attachment

Cacheing these 8 bytes (including padding) reduces the number of times
that anv_cmd_buffer_clear_attachments needs to loop over the pass's
attachments.