i965: Implement support for overriding the texture target in brw_emit_surface_state.

i965: Implement support for overriding the texture target in brw_emit_surface_state.

This allows the caller to bind a miptree using a texture target other
than the one it it was created with.  The code should work even if the
memory layouts of the specified and original targets don't match, as
long as the caller only intends to access a single slice of the
miptree structure.

This will be exploited by the next commit in order to support
non-coherent framebuffer fetch of a single layer of a 3D texture
(since some generations lack the minimum array element control for 3D
textures bound to the sampler unit), and multiple layers of a 1D array
texture (since binding it as an actual 1D array texture would require
state-dependent recompiles because the same shader couldn't
simultaneously work for 1D and 2D array textures due to the different
texel fetch coordinate ordering).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>