anv: Clamp depth buffer dimensions to be at least 1.

anv: Clamp depth buffer dimensions to be at least 1.

When there are no framebuffer attachments, fb->width and fb->height will
be 0.  Subtracting 1 results in 4294967295 which is too large for the
field, causing genxml assertions when trying to create the packet.

In this case, we can just program it to 1.

Caught by dEQP-VK.tessellation.tesscoord.triangles_equal_spacing.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>