iris: Compact binding tables

iris: Compact binding tables

Change the iris_binding_table to keep track of what surfaces are
actually going to be used, then assign binding table indices just for
those.  Reducing unused bytes on those are valuable because we use a
reduced space for those tables in Iris.

The rest of the driver can go from "group indices" (i.e. UBO #2) to
BTI and vice-versa using helper functions.  The value
IRIS_SURFACE_NOT_USED is returned to indicate a certain group index is
not used or a certain BTI is not valid.

The environment variable INTEL_DISABLE_COMPACT_BINDING_TABLE can be
set to skip compacting binding table.

v2: (all from Ken)
    Use BITFIELD64_MASK helper. Improve comments.
    Assert all group is marked as used when we have indirects.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>