i965/tex: Unify the TexImage and TexSubImage code

i965/tex: Unify the TexImage and TexSubImage code

It's nearly the same so there's no good reason why it can't be in a
common function.  The one difference is that _mesa_store_teximage
calls AllocTextureImageBuffer for us, while _mesa_store_texsubimage
doesn't, but we don't need that anyway - intelTexImage already does it.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>