i965: Removed the field etc_format from the struct intel_mipmap_tree

i965: Removed the field etc_format from the struct intel_mipmap_tree

After the previous changes to emulate the ETC/EAC formats using the
secondary shadow miptree, the etc_format field of the intel_mipmap_tree
struct became redundant and the remaining check that used it has been
replaced. (Nanley Chery)

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>