Revert "intel: Always allocate miptrees from level 0, not tObj->BaseLevel."

Revert "intel: Always allocate miptrees from level 0, not tObj->BaseLevel."

This reverts commit 7ce6517f3ac41bf770ab39aba4509d4f535ef663.
This reverts commit d60145d06d999c5c76000499e6fa9351e11d17fa.

I was wrong about which generations supported baselevel adjustment --
it's just gen4, nothing earlier.  This meant that i915 would have
never used the mag filter when baselevel != 0.  Not a severe bug, but
not an intentional regression.  I think we can fix the performance
issue another way.