anv: fix multiview for clear commands

anv: fix multiview for clear commands

According to the VK_KHX_multiview spec:

"Multiview causes all drawing and clear commands in the subpass to
behave as if they were broadcast to each view, where each view is
represented by one layer of the framebuffer attachments."

This adds support for multiview clears, which were missing in the
initial implementation.

v2 (Jason):
  - split multiview from regular case
  - Use for_each_bit() macro

Fixes new CTS multiview tests:
dEQP-VK.multiview.clear_attachments.*

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>