intel: Silence several "warning: unused parameter"

intel: Silence several "warning: unused parameter"

Also clean-up some of the naming, etc. in
intel_buffer_object_purgeable.  'intel' is usually used as the name of
an intel_context pointer, and intel_obj is usually used as the name of
an intel_*_obj pointer.  These changes were suggested by Eric Anholt.

Reviewed-by: Eric Anholt <eric@anholt.net>