anv: Don't resolve or ambiguate non-existent layers

anv: Don't resolve or ambiguate non-existent layers

The previous code was trying to avoid non-existent layers by taking a
MAX with anv_image_aux_layers.  Unfortunately, it wasn't taking into
account that layer_count starts at base_layer which may not be zero.
Instead, we need to subtract base_layer from anv_image_aux_layers with
a guard against roll-over.

Fixes: de3be6180169f9 "anv/cmd_buffer: Rework aux tracking"
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>