i965: Stop allocating miptrees with first_level != 0.

i965: Stop allocating miptrees with first_level != 0.

If the caller shows up with GL_BASE_LEVEL != 0, it doesn't mean that the
texture will over the course of its lifetime have that nonzero baselevel,
it means that the caller is filling the texture from the bottom up for
some reason (one could imagine demand-loading detailed texture layers at
runtime, for example).  If we allocate from just the current baselevel, it
means when they come along with the next level up, we'll have to allocate
a new miptree and copy all of our bits out of the first miptree.

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