i965: Always look up from the object's mt when setting up texturing state.

i965: Always look up from the object's mt when setting up texturing state.

We know that the object's mt is equal to the firstimage's mt because it's
gone through intel_finalize_mipmap_tree().  Saves a lookup of firstimage
on pre-gen7.

v2: Merge in the warning fix that appeared later in the series (noted by
    Chad)

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>